Screenplay Summarization Using Latent Narrative Structure

 
Screenplay Summarization Using Latent Narrative Structure
Screenplay Summarization Using Latent Narrative Structure
       Pinelopi Papalampidi1 Frank Keller1 Lea Frermann2 Mirella Lapata1
                    1 Institute for Language, Cognition and Computation

                        School of Informatics, University of Edinburgh
                       2 School of Computing and Information Systems

                                    University of Melbourne
                 p.papalampidi@sms.ed.ac.uk, keller@inf.ed.ac.uk,
                  lea.frermann@unimelb.edu.au, mlap@inf.ed.ac.uk

                     Abstract                                Victim: Mike Kimble, found in a Body Farm. Died 6
                                                                                                                  Setup
                                                             hours ago, unknown cause of death.
                                                             CSI discover cow tissue in Mike's body.                Opportunity
    Most general-purpose extractive summariza-               Cross-contamination is suggested. Probable
    tion models are trained on news articles, which          cause of death: Mike's house has been set on
                                                                                                                   New
                                                             fire. CSI finds blood: Mike was murdered, fire was
    are short and present all important information                                                              Situation
                                                             a cover up. First suspects: Mike's fiance, Jane
    upfront. As a result, such models are biased by          and her ex-husband, Russ.
                                                             CSI finds photos in Mike's house of Jane's               Change of
    position and often perform a smart selection             daughter, Jodie, posing naked.                            Plans
    of sentences from the beginning of the doc-              Mike is now a suspect of abusing Jodie. Russ
                                                                                                                 Progress
                                                             allows CSI to examine his gun.
    ument. When summarizing long narratives,                 CSI discovers that the bullet that killed Mike
                                                                                                                      Point of
    which have complex structure and present in-             was made of frozen beef that melt inside him.
                                                                                                                     no Return
                                                             They also find beef in Russ' gun.
    formation piecemeal, simple position heuris-             Russ confesses that he knew that Mike was
    tics are not sufficient. In this paper, we pro-                                                              Complications
                                                             abusing Jody, so he confronted and killed him.
    pose to explicitly incorporate the underlying            Russ is given bail, since no jury would convict           Major
                                                             a protective father.                                     Setback
    structure of narratives into general unsuper-            CSI discovers that the naked photos were taken
                                                                                                                 The final push
    vised and supervised extractive summarization            on a boat, which belongs to Russ.
                                                             CSI discovers that it was Russ who was
    models. We formalize narrative structure in              abusing his daughter based on fluids found in
                                                                                                                       Climax
    terms of key narrative events (turning points)           his sleeping bag and later killed Mike who tried
                                                             to help Jodie.
    and treat it as latent in order to summarize             Russ receives a mandatory life sentence.            Aftermath
    screenplays (i.e., extract an optimal sequence
    of scenes). Experimental results on the CSI              Figure 1: Example of narrative structure for episode
    corpus of TV screenplays, which we augment               “Burden of Proof” from TV series Crime Scene Inves-
    with scene-level summarization labels, show              tigation (CSI); turning points are highlighted in color.
    that latent turning points correlate with im-
    portant aspects of a CSI episode and improve
    summarization performance over general ex-               ements of a story in the beginning and support-
    tractive algorithms, leading to more complete            ing material and secondary details afterwards. The
    and diverse summaries.                                   rigid structure of news articles is expedient since
                                                             important passages can be identified in predictable
1   Introduction                                             locations (e.g., by performing a “smart selection”
Automatic summarization has enjoyed renewed                  of sentences from the beginning of the document)
interest in recent years thanks to the popular-              and the structure itself can be explicitly taken into
ity of modern neural network-based approaches                account in model design (e.g., by encoding the rel-
(Cheng and Lapata, 2016; Nallapati et al., 2016,             ative and absolute position of each sentence).
2017; Zheng and Lapata, 2019) and the avail-                    In this paper we are interested in summarizing
ability of large-scale datasets containing hundreds          longer narratives, i.e., screenplays, whose form
of thousands of document–summary pairs (Sand-                and structure is far removed from newspaper ar-
haus, 2008; Hermann et al., 2015; Grusky et al.,             ticles. Screenplays are typically between 110 and
2018; Narayan et al., 2018; Fabbri et al., 2019; Liu         120 pages long (20k words), their content is bro-
and Lapata, 2019). Most efforts to date have con-            ken down into scenes, which contain mostly dia-
centrated on the summarization of news articles              logue (lines the actors speak) as well as descrip-
which tend to be relatively short and formulaic              tions explaining what the camera sees. Moreover,
following an “inverted pyramid” structure which              screenplays are characterized by an underlying
places the most essential, novel and interesting el-         narrative structure, a sequence of events by which

                                                        1920
       Proceedings of the 58th Annual Meeting of the Association for Computational Linguistics, pages 1920–1933
                          July 5 - 10, 2020. c 2020 Association for Computational Linguistics
Screenplay Summarization Using Latent Narrative Structure
Screenplay      Latent Narrative Structure
                                                             ermann et al., 2018) which revolves around a team
                          TP1: Introduction                  of forensic investigators solving criminal cases.
                          TP2: Goal definition
                                                             Such programs have a complex but well-defined
                                                             structure: they open with a crime, the crime scene
                          TP3: Commitment
                                                             is examined, the victim is identified, suspects are
                          TP4: Setback                       introduced, forensic clues are gathered, suspects
                          TP5: Ending                        are investigated, and finally the case is solved.
                                                                In this work, we adapt general-purpose extrac-
                                                             tive summarization algorithms (Nallapati et al.,
          Summary scenes                                     2017; Zheng and Lapata, 2019) to identify infor-
                                          Video summary
                             relevant                        mative scenes in screenplays and instill in them
                              to TP2
                                                             knowledge about narrative film structure (Hauge,
                                                             2017; Cutting, 2016; Freytag, 1896). Specifically,
                                                             we adopt a scheme commonly used by screen-
                             irrelevant                      writers as a practical guide for producing success-
                                                             ful screenplays. According to this scheme, well-
                             relevant
                              to TP5
                                                             structured stories consist of six basic stages which
                                                             are defined by five turning points (TPs), i.e., events
                                                             which change the direction of the narrative, and
Figure 2: We first identify scenes that act as turning       determine the story’s progression and basic the-
points (i.e., key events that segment the story into sec-    matic units. In Figure 1, TPs are highlighted for
tions). We next create a summary by selecting informa-       a CSI episode. Although the link between turning
tive scenes, i.e.,semantically related to turning points.    points and summarization has not been previously
                                                             made, earlier work has emphasized the importance
a story is defined (Cutting, 2016), and by the               of narrative structure for summarizing books (Mi-
story’s characters and their roles (Propp, 1968).            halcea and Ceylan, 2007) and social media content
Contrary to news articles, the gist of the story in a        (Kim and Monroy-Hernández, 2015). More re-
screenplay is not disclosed at the start, information        cently, Papalampidi et al. (2019) have shown how
is often revealed piecemeal; characters evolve and           to identify turning points in feature-length screen-
their actions might seem more or less important              plays by projecting synopsis-level annotations.
over the course of the narrative. From a modeling               Crucially, our method does not involve man-
perspective, obtaining training data is particularly         ually annotating turning points in CSI episodes.
problematic: even if one could assemble screen-              Instead, we approximate narrative structure au-
plays and corresponding summaries (e.g., by min-             tomatically by pretraining on the annotations of
ing IMDb or Wikipedia), the size of such a corpus            the TRIPOD dataset of Papalampidi et al. (2019)
would be at best in the range of a few hundred               and employing a variant of their model. We find
examples not hundreds of thousands. Also note                that narrative structure representations learned on
that genre differences might render transfer learn-          their dataset (which was created for feature-length
ing (Pan and Yang, 2010) difficult, e.g., a model            films), transfer well across cinematic genres and
trained on movie screenplays might not generalize            computational tasks. We propose a framework for
to sitcoms or soap operas.                                   end-to-end training in which narrative structure is
   Given the above challenges, we introduce a                treated as a latent variable for summarization. We
number of assumptions to make the task feasible.             extend the CSI dataset (Frermann et al., 2018) with
Firstly, our goal is to produce informative sum-             binary labels indicating whether a scene should be
maries, which serve as a surrogate to reading the            included in the summary and present experiments
full script or watching the entire film. Secondly,           with both supervised and unsupervised summa-
we follow Gorinski and Lapata (2015) in con-                 rization models. An overview of our approach is
ceptualizing screenplay summarization as the task            shown in Figure 2.
of identifying a sequence of informative scenes.               Our contributions can be summarized as fol-
Thirdly, we focus on summarizing television pro-             lows: (a) we develop methods for instilling knowl-
grams such as CSI: Crime Scene Investigation (Fr-            edge about narrative structure into generic su-

                                                          1921
pervised and unsupervised summarization algo-            search on narrative structure analysis (Cutting,
rithms; (b) we provide a new layer of annotations        2016; Hauge, 2017), screenplay summarization
for the CSI corpus, which can be used for research       (Gorinski and Lapata, 2015), and neural network
in long-form summarization; and (c) we demon-            modeling (Dong, 2018). We focus on extractive
strate that narrative structure can facilitate screen-   summarization and our goal is to identify an op-
play summarization; our analysis shows that key          timal sequence of key events in a narrative. We
events identified in the latent space correlate with     aim to create summaries which re-tell the plot of a
important summary content.                               story in a concise manner. Inspired by recent neu-
                                                         ral network-based approaches (Cheng and Lapata,
2   Related Work                                         2016; Nallapati et al., 2017; Zhou et al., 2018;
                                                         Zheng and Lapata, 2019), we develop supervised
A large body of previous work has focused on the         and unsupervised models for our summarization
computational analysis of narratives (Mani, 2012;        task based on neural representations of scenes
Richards et al., 2009). Attempts to analyze how          and how these relate to the screenplay’s narra-
stories are written have been based on sequences         tive structure. Contrary to most previous work
of events (Schank and Abelson, 1975; Chambers            which has focused on characters, we select sum-
and Jurafsky, 2009), plot units (McIntyre and Lap-       mary scenes based on events and their importance
ata, 2010; Goyal et al., 2010; Finlayson, 2012) and      in the story. Our definition of narrative structure
their structure (Lehnert, 1981; Rumelhart, 1980),        closely follows Papalampidi et al. (2019). How-
as well as on characters or personas in a narrative      ever, the model architectures we propose are gen-
(Black and Wilensky, 1979; Propp, 1968; Bam-             eral and could be adapted to different plot analysis
man et al., 2014, 2013; Valls-Vargas et al., 2014)       schemes (Field, 2005; Vogler, 2007). To overcome
and their relationships (Elson et al., 2010; Agarwal     the difficulties in evaluating summaries for longer
et al., 2014; Srivastava et al., 2016).                  narratives, we also release a corpus of screenplays
   As mentioned earlier, work on summarization           with scenes labeled as important (summary wor-
of narratives has had limited appeal, possibly due       thy). Our annotations augment an existing dataset
to the lack of annotated data for modeling and           based on CSI episodes (Frermann et al., 2018),
evaluation. Kazantseva and Szpakowicz (2010)             which was originally developed for incremental
summarize short stories based on importance cri-         natural language understanding.
teria (e.g., whether a segment contains protagonist
or location information); they create summaries to       3     Problem Formulation
help readers decide whether they are interested in
reading the whole story, without revealing its plot.     Let D denote a screenplay consisting of a se-
Mihalcea and Ceylan (2007) summarize books               quence of scenes D = {s1 , s2 , . . . , sn }. Our aim is
with an unsupervised graph-based approach op-            to select a subset D 0 = {si , . . . , sk } consisting of
erating over segments (i.e., topical units). Their       the most informative scenes (where k < n). Note
algorithm first generates a summary for each seg-        that this definition produces extractive summaries;
ment and then an overall summary by collecting           we further assume that selected scenes are pre-
sentences from the individual segment summaries.         sented according to their order in the screenplay.
   Focusing on screenplays, Gorinski and Lapata          We next discuss how summaries can be created us-
(2015) generate a summary by extracting an opti-         ing both unsupervised and supervised approaches,
mal chain of scenes via a graph-based approach           and then move on to explain how these are adapted
centered around the main characters. In a sim-           to incorporate narrative structure.
ilar fashion, Tsoneva et al. (2007) create video
summaries for TV series episodes; their algorithm        3.1    Unsupervised Screenplay Summarization
ranks sub-scenes in terms of importance using fea-       Our unsupervised model is based on an extension
tures based on character graphs and textual cues         of T EXT R ANK (Mihalcea and Tarau, 2004; Zheng
available in the subtitles and movie scripts. Vicol      and Lapata, 2019), a well-known algorithm for ex-
et al. (2018) introduce the MovieGraphs dataset,         tractive single-document summarization. In our
which also uses character-centered graphs to de-         setting, a screenplay is represented as a graph, in
scribe the content of movie video clips.                 which nodes correspond to scenes and edges be-
   Our work synthesizes various strands of re-           tween scenes si and s j are weighted by their simi-

                                                     1922
larity ei j . A node’s centrality (importance) is mea-       We encode the screenplay with a BiLSTM net-
sured by computing its degree:                               work and obtain contextualized representations s0i
                                                             for scenes si by concatenating the hidden layers of
          centrality(si ) = λ1 ∑ ei j + λ2 ∑ ei j     (1)                    →
                                                                             −                    ←
                                                                                                  −
                                                             the forward hi and backward hi LSTM, respec-
                               ji                              →
                                                                            −    ←−
                                                             tively: s0i = [ hi ; hi ]. The vector s0i therefore repre-
where λ1 +λ2 = 1. The modification introduced in             sents the content of the ith scene.
Zheng and Lapata (2019) takes directed edges into               We also estimate the salience of scene si by
account, capturing the intuition that the centrality         measuring its similarity with a global screenplay
of any two nodes is influenced by their relative po-         content representation d. The latter is the weighted
sition. Also note that the edges of preceding and            sum of all scene representations s1 , s2 , . . . , sn . We
following scenes are differentially weighted by λ1           calculate the semantic similarity between s0i and d
and λ2 .                                                     by computing the element-wise dot product bi , co-
   Although earlier implementations of T EXT -               sine similarity ci , and pairwise distance ui between
R ANK (Mihalcea and Tarau, 2004) compute node                their respective vectors:
similarity based on symbolic representations such
                                                                                                     s0i · d
as tf*idf, we adopt a neural approach. Specifically,                       bi = s0i     d    ci =                     (2)
                                                                                                    s0i kdk
we obtain sentence representations based on a pre-
trained encoder. In our experiments, we rely on                                            s0i · d
                                                                             ui =                                     (3)
the Universal Sentence Encoder (USE; Cer et al.                                       max(ks0i k2 · kdk2 )
2018), however, other embeddings are possible.1
We represent a scene by the mean of its sentence             The salience vi of scene si is the concatenation of
representations and measure scene similarity ei j            the similarity metrics: vi = [bi ; ci ; ui ]. The content
using cosine.2 As in the original T EXT R ANK al-            vector s0i and the salience vector vi are concate-
gorithm (Mihalcea and Tarau, 2004), scenes are               nated and fed to a single neuron that outputs the
ranked based on their centrality and the M most              probability of a scene belonging to the summary.3
central ones are selected to appear in the summary.          3.3   Narrative Structure
3.2   Supervised Screenplay Summarization                    We now explain how to inject knowledge about
                                                             narrative structure into our summarization models.
Most extractive models frame summarization as
                                                             For both models, such knowledge is transferred
a classification problem. Following a recent ap-
                                                             via a network pre-trained on the TRIPOD4 dataset
proach (S UMMA RU NN ER; Nallapati et al. 2017),
                                                             introduced by Papalampidi et al. (2019). This
we use a neural network-based encoder to build
                                                             dataset contains 99 movies annotated with turning
representations for scenes and apply a binary clas-
                                                             points. TPs are key events in a narrative that define
sifier over these to predict whether they should
                                                             the progression of the plot and occur between con-
be in the summary. For each scene si ∈ D , we
                                                             secutive acts (thematic units). It is often assumed
predict a label yi ∈ {0, 1} (where 1 means that
                                                             (Cutting, 2016) that there are six acts in a film
si must be in the summary) and assign a score
                                                             (Figure 1), each delineated by a turning point (ar-
p(yi |si , D , θ) quantifying si ’s relevance to the sum-
                                                             rows in the figure). Each of the five TPs has also a
mary (θ denotes model parameters). We assem-
                                                             well-defined function in the narrative: we present
ble a summary by selecting M sentences with the
                                                             each TP alongside with its definition as stated in
top p(1|si , D , θ).
                                                             screenwriting theory (Hauge, 2017) and adopted
   We calculate sentence representations via the
                                                             by Papalampidi et al. (2019) in Table 1 (see Ap-
pre-trained USE encoder (Cer et al., 2018); a scene
                                                             pendix A for a more detailed description of nar-
is represented as the weighted sum of the repre-
                                                             rative structure theory). Papalampidi et al. (2019)
sentations of its sentences, which we obtain from
                                                             identify scenes in movies that correspond to these
a BiLSTM equipped with an attention mechanism.
                                                             key events as a means for analyzing the narrative
Next, we compute richer scene representations by
modeling surrounding context of a given scene.                  3 Aside from salience and content, Nallapati et al. (2017)
                                                             take into account novelty and position-related features. We
   1 USE  performed better than BERT in our experiments.     ignore these as they are specific to news articles and denote
   2 We found cosine to be particularly effective with USE   the modified model as S UMMA RU NN ER *.
representations; other metrics are also possible.                4 https://github.com/ppapalampidi/TRIPOD

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Turning Point                     Definition                Summarization with Narrative Structure).5 We
                        Introductory event that occurs after
TP1: Opportunity        the presentation of the story setting.   first present an unsupervised variant which mod-
                        Event where the main goal of the         ifies the computation of scene centrality in the di-
TP2: Change of Plans story is defined.
                                                                 rected version of T EXT R ANK (Equation (1)).
                        Event that pushes the main charac-
TP3: Point of No Return ter(s) to fully commit to their goal.       Specifically, we use the pre-trained network de-
                        Event where everything falls apart       scribed in Section 3.3 to obtain TP-specific at-
TP4: Major Setback      (temporarily or permanently).            tention distributions. We then select an overall
                        Final event of the main story, mo-
TP5: Climax             ment of resolution.                      score fi for each scene (denoting how likely it is
                                                                 to act as a TP). We set fi = max j∈[1,5] pi j , i.e., to
Table 1: Turning points and their definitions as given
in Papalampidi et al. (2019)                                     the pi j value that is highest across TPs. We incor-
                                                                 porate these scores into centrality as follows:
structure of movies. They collect sentence-level                   centrality(si )=λ1 ∑ (ei j + f j )+λ2 ∑ (ei j + fi ) (6)
TP annotations for plot synopses and subsequently                                      ji
project them via distant supervision onto screen-
plays, thereby creating silver-standard labels. We               Intuitively, we add the f j term in the forward sum
utilize this silver-standard dataset in order to pre-            in order to incrementally increase the centrality
train a network which performs TP identification.                scores of scenes as the story moves on and we en-
                                                                 counter more TP events (i.e., we move to later sec-
TP Identification Network We first encode                        tions in the narrative). At the same time, we add
screenplay scenes via a BiLSTM equipped with an                  the fi term in the backward sum in order to also
attention mechanism. We then contextualize them                  increase the scores of scenes identified as TPs.
with respect to the whole screenplay via a second
                                                                 Supervised S UMMER We also propose a su-
BiLSTM. Next, we compute topic-aware scene
                                                                 pervised variant of S UMMER following the basic
representations ti via a context interaction layer
                                                                 model formulation in Section 3.3. We still repre-
(CIL) as proposed in Papalampidi et al. (2019).
                                                                 sent a scene as the concatenation of a content vec-
CIL is inspired by traditional segmentation ap-
                                                                 tor s0 and salience vector v0 , which serve as input
proaches (Hearst, 1997) and measures the seman-
                                                                 to a binary classifier. However, we now modify
tic similarity of the current scene with a preceding
                                                                 how salience is determined; instead of comput-
and following context window in the screenplay.
                                                                 ing a general global content representation d for
Hence, the topic-aware scene representations also
                                                                 the screenplay, we identify a sequence of TPs and
encode the degree to which each scene acts as a
                                                                 measure the semantic similarity of each scene with
topic boundary in the screenplay.
                                                                 this sequence. Our model is depicted in Figure 3.
   In the final layer, we employ TP-specific atten-
                                                                    We utilize the pre-trained TP network (Fig-
tion mechanisms to compute the probability pi j
                                                                 ures 3(a) and (b)) to compute sparse attention
that scene ti represents the jth TP in the screen-
                                                                 scores over scenes. In the supervised setting,
play. Note that we expect the TP-specific atten-
                                                                 where gold-standard binary labels provide a train-
tion distributions to be sparse, as there are only
                                                                 ing signal, we fine-tune the network in an end-to-
a few scenes which are relevant for a TP (recall
                                                                 end fashion on summarization (Figure 3(c)). We
that TPs are boundary scenes between sections).
                                                                 compute the TP representations via the attention
To encourage sparsity, we add a low temperature
                                                                 scores; we calculate a vector t p j as the weighted
value τ (Hinton et al., 2015) to the softmax part of
                                                                 sum of all topic-aware scene representations t pro-
the attention mechanisms:
                                                                 duced via CIL: t p j = ∑i∈[1,N] pi j ti , where N is the
        gi j = tanh(W j ti + b j ),    g j ∈ [−1, 1]     (4)     number of scenes in a screenplay. In practice, only
                                         T
                                                                 a few scenes contribute to t p j due to the τ param-
                   exp(gi j /τ)                                  eter in the softmax function (Equation (5)).
        pi j =    T                ,    ∑ pi j = 1       (5)
                 ∑t=1 exp(gt j /τ)      i=1                         A TP-scene interaction layer measures the se-
                                                                 mantic similarity between scenes ti and latent TP
where W j , b j represent the trainable weights of the
                                                                 representations t p j (Figure 3(c)). Intuitively, a
attention layer of the jth TP.
                                                                 complete summary should contain scenes which
Unsupervised S UMMER We now introduce                               5 We make our code publicly available at https://
our model, S UMMER (short for Screenplay                         github.com/ppapalampidi/SUMMER.

                                                            1924
(c): Summary scenes prediction                   when a TP should occur (e.g., the Opportunity oc-
                    Final scene
                                                                                       curs after the first 10% of a screenplay, Change of

                                           ..
                                                             y1, ..., yk, ..., yM
                  representations                                                      Plans is approximately 25% in). It is reasonable to
                                                                                       discourage t p distributions to deviate drastically
     TP-scene                                                                          from these expected positions. Focal regulariza-

                                                       ..
                                 ..
    Interaction
       Layer                                                                           tion F minimizes the KL divergence DKL between
                           Salience wrt           Content                              each TP attention distribution t pi and its expected
                             plotline
                                                                                       position distribution thi :
                        (b): Narrative structure prediction
      tp1           tp2           tp3            tp4          tp5                                    F=    ∑        DKL (t pi kthi )    (8)
                                                                                                          i∈[1,5]

                                                                                       The final loss L is the weighted sum of all three
            TP1         TP2       TP3           TP4         TP5
                                                                                       components, where a, b are fixed during training:
                                                                                       L = BCE + aO + bF.
                   t1      ...   tk     ...      tM
                                                                                       4    Experimental Setup
                   Content Interaction Layer
                                                                                       Crime Scene Investigation Dataset We per-
                  s'1     ...    s'k    ...     s'M
                                                                                       formed experiments on an extension of the CSI
                        Screenplay Encoder
                             (BiLSTM)
                                                                                       dataset6 introduced by Frermann et al. (2018). It
                                                                                       consists of 39 CSI episodes, each annotated with
                   s1      ...   sk      ...    sM
                                        (a): Scene encoding                            word-level labels denoting whether the perpetra-
                                                                                       tor is mentioned in the utterances characters speak.
Figure 3: Overview of S UMMER. We use one
                                                                                       We further collected scene-level binary labels in-
TP-specific attention mechanism per turning point in
order to acquire TP-specific distributions over scenes.                                dicating whether episode scenes are important and
We then compute the similarity between TPs and con-                                    should be included in a summary. Three human
textualized scene representations. Finally, we perform                                 judges performed the annotation task after watch-
max pooling over TP-specific similarity vectors and                                    ing the CSI episodes scene-by-scene. To facilitate
concatenate the final similarity representation with the                               the annotation, judges were asked to indicate why
contextualized scene representation.                                                   they thought a scene was important, citing the fol-
                                                                                       lowing reasons: it revealed (i) the victim, (ii) the
are related to at least one of the key events in                                       cause of death, (iii) an autopsy report, (iv) crucial
the screenplay. We calculate the semantic similar-                                     evidence, (v) the perpetrator, and (vi) the motive or
ity vi j of scene ti with TP t p j as in Equations (2)                                 the relation between perpetrator and victim. An-
and (3). We then perform max pooling over vec-                                         notators were free to select more than one or none
tors vi1 , . . . , viT , where T is the number of TPs                                  of the listed reasons where appropriate. We can
(i.e., five) and calculate a final similarity vector v0i                               think of these reasons as high-level aspects a good
for the ith scene.                                                                     summary should cover (for CSI and related crime
   The model is trained end-to-end on the summa-                                       series). Annotators were not given any informa-
rization task using BCE, the binary cross-entropy                                      tion about TPs or narrative structure; the annota-
loss function. We add an extra regularization term                                     tion was not guided by theoretical considerations,
to this objective to encourage the TP-specific at-                                     rather our aim was to produce useful CSI sum-
tention distributions to be orthogonal (since we                                       maries. Table 2 presents the dataset statistics (see
want each attention layer to attend to different                                       also Appendix B for more detail).
parts of the screenplay). We thus maximize the
                                                                                       Implementation Details In order to set the hy-
Kullback-Leibler (KL) divergence DKL between
                                                                                       perparameters of all proposed networks, we used
all pairs of TP attention distributions t pi , i ∈ [1, 5]:
                                                                                       a small development set of four episodes from the
                                                       1                               CSI dataset (see Appendix B for details). After ex-
   O=       ∑            ∑        log                                       (7)       perimentation, we set the temperature τ of the soft-
         i∈[1,5] j∈[1,5], j6=i           DKL t pi t p j + ε
                                                                                       max layers for the TP-specific attentions (Equa-
Furthermore, we know from screenwriting theory                                         tion (5)) to 0.01. Since the binary labels in the
(Hauge, 2017) that there are rules of thumb as to                                          6 https://github.com/EdinburghNLP/csi-corpus

                                                                                    1925
overall                                  Model                                             F1
    episodes                                       39             Lead 30%                                         30.66
    scenes                                       1544             Last 30%                                         39.85
    summary scenes                                454             Mixed 30%                                        34.32
                        per episode                               T EXT R ANK, undirected, tf*idf                  32.11
    scenes                                39.58 (6.52)            T EXT R ANK, directed, neural                    41.75
    crime-specific aspects                  5.62 (0.24)           T EXT R ANK, directed, expected TP positions     41.05
    summary scenes                         11.64 (2.98)           S CENE S UM, directed, character-based weights   42.02
    summary scenes (%)                     29.75 (7.35)           S UMMER                                          44.70
    sentences                          822.56 (936.23)
                                                                  Table 3: Unsupervised screenplay summarization.
    tokens                             13.27k (14.67k)
                     per episode scene                                                                  Coverage # scenes
    sentences                            20.78 (35.61)                                           F1
                                                                                                        of aspects per TP
    tokens                             335.19 (547.61)           Lead 30%                       30.66        –         –
    tokens per sentence                  16.13 (16.32)           Last 30%                       39.85        –         –
                                                                 Mixed 30%                      34.32        –         –
    Table 2: CSI dataset statistics; means and (std).
                                                                 S UMMA RU NN ER *              48.56        –         –
                                                                 S CENE S UM                    47.71        –         –
                                                                 S UMMER, fixed one-hot TPs     46.92     63.11      1.00
supervised setting are imbalanced, we apply class                S UMMER, fixed distributions   47.64     67.01      1.05
weights to the binary cross-entropy loss of the re-              S UMMER, −P, −R                51.93     44.48      1.19
spective models. We weight each class by its in-                 S UMMER, −P, +R                49.98     51.96      1.14
                                                                 S UMMER, +P, −R                50.56     62.35      3.07
verse frequency in the training set. Finally, in su-             S UMMER, +P, +R                52.00     70.25      1.20
pervised S UMMER, where we also identify the nar-
                                                             Table 4: Supervised screenplay summarization; for in
rative structure of the screenplays, we consider as          S UMMER variants, we also report the percentage of as-
key events per TP the scenes that correspond to an           pect labels covered by latent TP predictions.
attention score higher than 0.05. More implemen-
tation details can be found in Appendix C.                   tion in Gorinski and Lapata (2015):
   As shown in Table 2, the gold-standard sum-
maries in our dataset have a compression rate of                                  ∑c∈C [c ∈ S ∪ main(C)]
                                                                           ci =                                        (9)
approximately 30%. During inference, we select                                          ∑c∈C [c ∈ S]
the top M scenes as the summary, such that they
correspond to 30% of the length of the episode.              where S is the set of all characters participating in
                                                             scene si , C is the set of all characters participat-
5   Results and Analysis                                     ing in the screenplay and main(C) are all the main
                                                             characters of the screenplay. We retrieve the set
Is Narrative Structure Helpful? We perform                   of main characters from the IMDb page of the re-
10-fold cross-validation and evaluate model per-             spective episode. We also note that human agree-
formance in terms of F1 score. Table 3 sum-                  ment for our task is 79.26 F1 score, as measured
marizes the results of unsupervised models. We               on a small subset of the corpus.
present the following baselines: Lead 30% se-                   As shown in Table 3, S UMMER achieves the
lects the first 30% of an episode as the summary,            best performance (44.70 F1 score) among all mod-
Last 30% selects the last 30%, and Mixed 30%,                els and is superior to an equivalent model which
randomly selects 15% of the summary from                     uses expected TP positions or a character-based
the first 30% of an episode and 15% from the                 representation. This indicates that the pre-trained
last 30%. We also compare S UMMER against                    network provides better predictions for key events
T EXT R ANK based on tf*idf (Mihalcea and Ta-                than position and character heuristics, even though
rau, 2004), the directed neural variant described            there is a domain shift from Hollywood movies
in Section 3.1 without any TP information, a                 in the TRIPOD corpus to episodes of a crime
variant where TPs are approximated by their ex-              series in the CSI corpus. Moreover, we find
pected position as postulated in screenwriting the-          that the directed versions of T EXT R ANK are bet-
ory, and a variant that incorporates information             ter at identifying important scenes than the undi-
about characters (Gorinski and Lapata, 2015) in-             rected version. We found that performance peaks
stead of narrative structure. For the character-             with λ1 = 0.7 (see Equation (6)), indicating that
based T EXT R ANK, called S CENE S UM, we substi-            higher importance is given to scenes as the story
tute the fi , f j scores in Equation (6) with character-     progresses (see Appendix D for experiments with
related importance scores ci similar to the defini-          different λ values).

                                                          1926
In Table 4, we report results for supervised             or provides important evidence). Specifically, we
models. Aside from the various baselines in the              compute the extent to which these aspects overlap
first block of the table, we compare the neural              with the TPs predicted by S UMMER as:
extractive model S UMMA RU NN ER *7 (Nallapati
                                                                       ∑Ai ∈A ∑T Pj ∈T P [dist(T Pj ,Ai )≤1]
et al., 2017) presented in Section 3.2 with sev-                  C=                                           (10)
eral variants of our model S UMMER. We exper-                                           |A|
imented with randomly initializing the network                where A is the set of all aspect scenes, |A| is the
for TP identification (−P) and with using a pre-             number of aspects, T P is the set of scenes inferred
trained network (+P). We also experimented with              as TPs by the model, Ai and T Pj are the subsets
removing the regularization terms, O and F (Equa-            of scenes corresponding to the ith aspect and jth
tions (7) and (8)) from the loss (−R). We as-                TP, respectively, and dist(T Pj , Ai ) is the minimum
sess the performance of S UMMER when we follow               distance between T Pj and Ai in number of scenes.
a two-step approach where we first predict TPs                  The proportion of aspects covered is given in
via the pre-trained network and then train a net-            Table 4, middle column. We find that coverage is
work on screenplay summarization based on fixed              relatively low (44.48%) for the randomly initial-
TP representations (fixed one-hot TPs), or alter-            ized S UMMER with no regularization. There is a
natively use expected TP position distributions as           slight improvement of 7.48% when we force the
postulated in screenwriting theory (fixed distribu-          TP-specific attention distributions to be orthogo-
tions). Finally, we incorporate character-based in-          nal and close to expected positions. Pre-training
formation into our baseline and create a supervised          and regularization provide a significant boost, in-
version of S CENE S UM. We now utilize the charac-           creasing coverage to 70.25%, while pre-trained
ter importance scores per scene (Equation (9)) as            S UMMER without regularization infers on aver-
attention scores – instead of using a trainable at-          age more scenes representative of each TP. This
tention mechanism – when computing the global                shows that the orthogonal constraint also encour-
screenplay representation d (Section 3.2).                   ages sparse attention distributions for TPs.
    Table 4 shows that all end-to-end S UMMER                   Table 5 shows the degree of association be-
variants outperform S UMMA RU NN ER *. The                   tween individual TPs and summary aspects (see
best result (52.00 F1 Score) is achieved by pre-             Appendix D for illustrated examples). We observe
trained S UMMER with regularization, outperform-             that Opportunity and Change of Plans are mostly
ing S UMMA RU NN ER * by an absolute difference              associated with information about the crime scene
of 3.44. The randomly initialized version with               and the victim, Climax is focused on the revelation
no regularization achieves similar performance               of the motive, while information relating to cause
(51.93 F1 score). For summarizing screenplays,               of death, perpetrator, and evidence is captured by
explicitly encoding narrative structure seems to             both Point of no Return and Major Setback. Over-
be more beneficial than general representations              all, the generic Hollywood-inspired TP labels are
of scene importance. Finally, two-step versions              adjusted to our genre and describe crime-related
of S UMMER perform poorly, which indicates that              key events, even though no aspect labels were pro-
end-to-end training and fine-tuning of the TP iden-          vided to our model during training.
tification network on the target dataset is crucial.
                                                             Do Humans Like the Summaries? We also
What Does the Model Learn? Apart from per-                   conducted a human evaluation experiment using
formance on summarization, we would also like to             the summaries created for 10 CSI episodes.8 We
examine the quality of the TPs inferred by S UM -            produced summaries based on the gold-standard
MER (supervised variant). Problematically, we do             annotations (Gold), S UMMA RU NN ER *, and the
not have any gold-standard TP annotation in the              supervised version of S UMMER. Since 30% of
CSI corpus. Nevertheless, we can implicitly assess           an episode results in lengthy summaries (15 min-
whether they are meaningful by measuring how                 utes on average), we further increased the com-
well they correlate with the reasons annotators cite         pression rate for this experiment by limiting each
to justify their decision to include a scene in the          summary to six scenes. For the gold standard con-
summary (e.g., because it reveals cause of death             dition, we randomly selected exactly one scene
   7 Our adaptation of S UMMA RU NN ER that considers con-      8 https://github.com/ppapalampidi/SUMMER/tree/
tent and salience vectors for scene selection.               master/video_summaries

                                                         1927
Turning Point                         Crime scene    Victim   Death Cause    Perpetrator     Evidence    Motive
    Opportunity                                 56.76     52.63         15.63         15.38          2.56      0.00
    Change of Plans                             27.03     42.11         21.88         15.38          5.13      0.00
    Point of no Return                           8.11     13.16          9.38         25.64         48.72      5.88
    Major Setback                                0.00      0.00          6.25         10.25         48.72     35.29
    Climax                                       2.70      0.00          6.25          2.56         23.08     55.88
                     Table 5: Percentage of aspect labels covered per TP for S UMMER, +P, +R.
    System               Crime scene   Victim   Death Cause    Perpetrator   Evidence   Motive     Overall    Rank
    S UMMA RU NN ER *          85.71    93.88         75.51         81.63       59.18    38.78      72.45      2.18
    S UMMER                    89.80    87.76         83.67         81.63       77.55    57.14      79.59      2.00
    Gold                       89.80    91.84         71.43         83.67       65.31    57.14      76.53      1.82
Table 6: Human evaluation: percentage of yes answers by AMT workers regarding each aspect in a summary. All
differences in (average) Rank are significant (p < 0.05, using a χ2 test).

per aspect. For S UMMA RU NN ER * and S UMMER              can be integrated with supervised and unsuper-
we selected the top six predicted scenes based             vised extractive summarization algorithms. Ex-
on their posterior probabilities. We then created          periments on the CSI corpus showed that this
video summaries by isolating and merging the se-           scheme transfers well to a different genre (crime
lected scenes in the raw video.                            investigation) and that utilizing narrative struc-
   We asked Amazon Mechanical Turk (AMT)                   ture boosts summarization performance, leading
workers to watch the video summaries for all sys-          to more complete and diverse summaries. Anal-
tems and rank them from most to least informa-             ysis of model output further revealed that latent
tive. They were also presented with six questions          events encapsulated by turning points correlate
relating to the aspects the summary was supposed           with important aspects of a CSI summary.
to cover (e.g., Was the victim revealed in the sum-           Although currently our approach relies solely
mary? Do you know who the perpetrator was?).               on textual information, it would be interesting to
They could answer Yes, No, or Unsure. Five work-           incorporate additional modalities such as video or
ers evaluated each summary.                                audio. Audiovisual information could facilitate
   Table 6 shows the proportion of times partic-           the identification of key events and scenes. Be-
ipants responded Yes for each aspect across the            sides narrative structure, we would also like to ex-
three systems. Although S UMMER does not im-               amine the role of emotional arcs (Vonnegut, 1981;
prove over S UMMA RU NN ER * in identifying ba-            Reagan et al., 2016) in a screenplay. An often in-
sic information (i.e., about the victim and perpe-         tegral part of a compelling story is the emotional
trator), it creates better summaries overall with          experience that is evoked in the reader or viewer
more diverse content (i.e., it more frequently in-         (e.g., somebody gets into trouble and then out of it,
cludes information about cause of death, evidence,         somebody finds something wonderful, loses it, and
and motive). This observation validates our as-            then finds it again). Understanding emotional arcs
sumption that identifying scenes that are semanti-         may be useful to revealing a story’s shape, high-
cally close to the key events of a screenplay leads        lighting important scenes, and tracking how the
to more complete and detailed summaries. Fi-               story develops for different characters over time.
nally, Table 6 also lists the average rank per system
                                                           Acknowledgments
(lower is better), which shows that crowdwork-
ers like gold summaries best, S UMMER is often             We thank the anonymous reviewers for their feed-
ranked second, followed by S UMMA RU NN ER * in            back. We gratefully acknowledge the support of
third place.                                               the European Research Council (Lapata; award
                                                           681760, “Translating Multiple Modalities into
6    Conclusions                                           Text”) and of the Leverhulme Trust (Keller; award
                                                           IAF-2017-019).
In this paper we argued that the underlying struc-
ture of narratives is beneficial for long-form sum-
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                                                      1930
Crime scene             Motive                                             Directed neural
                                                                            44       TextRank
                                                                            42       SUMMER
                      12.4% 10.9%
       Victim                                                               40
                  14.6%

                                                             F1 score (%)
                                                                            38
                                  36.1%
                  14.4%                    Evidence                         36
    Perpetrator           11.6%                                             34
                                                                            32
           Cause of death
                                                                                 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
                                                                                                       1

Figure 4: Average composition of a CSI summary           Figure 5: F1 score (%) for directed neural T EXT -
based on different crime-related aspects.                R ANK and S UMMER for unsupervised summarization
                                                         with respect to different λ1 values. Higher λ1 values
                                                         correspond to higher importance in the next context for
Hao Zheng and Mirella Lapata. 2019. Sentence cen-        the centrality computation of a current scene.
  trality revisited for unsupervised summarization.
  arXiv preprint arXiv:1906.03508.

Qingyu Zhou, Nan Yang, Furu Wei, Shaohan Huang,          B                  CSI Corpus
  Ming Zhou, and Tiejun Zhao. 2018. Neural docu-
  ment summarization by jointly learning to score and
  select sentences. arXiv preprint arXiv:1807.02305.
                                                         As described in Section 4, we collected aspect-
                                                         based summary labels for all episodes in the CSI
A   Narrative Structure Theory                           corpus. In Figure 4 we illustrate the average com-
                                                         position of a summary based on the different as-
The initial formulation of narrative structure was
                                                         pects seen in a crime investigation (e.g., crime
promoted by Aristotle, who defined the basic
                                                         scene, victim, cause of death, perpetrator, evi-
triangle-shaped plot structure, that has a beginning
                                                         dence). Most of these aspects are covered in
(protasis), middle (epitasis) and end (catastrophe)
                                                         10–15% of a summary, which corresponds to ap-
(Pavis, 1998). However, later theories argued that
                                                         proximately two scenes in the episode. Only
the structure of a play should be more complex
                                                         the “Evidence” aspect occupies a larger propor-
(Brink, 2011) and hence, other schemes (Freytag,
                                                         tion of the summary (36.1%) corresponding to
1896) were proposed with fine-grained stages and
                                                         five scenes. However, there exist scenes which
events defining the progression of the plot. These
                                                         cover multiple aspects (an as a result are anno-
events are considered as the precursor of turning
                                                         tated with more than one label) and episodes that
points, defined by Thompson (1999) and used in
                                                         do not include any scenes related to a specific as-
modern variations of screenplay theory. Turning
                                                         pect (e.g., if the murder was a suicide, there is no
points are narrative moments from which the plot
                                                         perpetrator).
goes in a different direction. By definition these
occur at the junctions of acts.                              We should note that Frermann et al. (2018) dis-
   Currently, there are myriad schemes describ-          criminate between different cases presented in the
ing the narrative structure of films, which are of-      same episode in the original CSI dataset. Specif-
ten used as a practical guide for screenwriters          ically, there are episodes in the dataset, where ex-
(Cutting, 2016). One variation of these modern           cept for the primary crime investigation case, a
schemes is adopted by Papalampidi et al. (2019),         second one is presented occupying a significantly
who focus on the definition of turning points and        smaller part of the episode. Although in the origi-
demonstrate that such events indeed exist in films       nal dataset, there are annotations available indicat-
and can be automatically identified. According           ing which scenes refer to each case, we assume no
to the adopted scheme (Hauge, 2017), there are           such knowledge treating the screenplay as a single
six stages (acts) in a film, namely the setup, the       unit — most TV series and movies contain sub-
new situation, progress, complications and higher        stories. We also hypothesize that the latent iden-
stakes, the final push and the aftermath, separated      tified TP events in S UMMER should relate to the
by the five turning points presented in Table 1.         primary case.

                                                      1931
Figure 6: Examples of inferred TPs alongside with gold-standard aspect-based summary labels in CSI episodes at
test time. The TP events are identified in the latent space for the supervised version of S UMMER (+P, +R).

C   Implementation Details                               the preceding ones, since λ1 and λ2 are bounded
                                                         (λ1 + λ2 = 1).
In all unsupervised versions of T EXT R ANK and
                                                            We observe that performance increases when
S UMMER we used a threshold h equal to 0.2 for re-
                                                         higher importance is attributed to screenplay
moving weak edges from the corresponding fully
                                                         scenes as the story moves on (λ1 > 0.5), whereas
connected screenplay graphs. For the supervised
                                                         for extreme cases (λ1 → 1), where only the later
version of S UMMER, where we use additional reg-
                                                         parts of the story are considered, performance
ularization terms in the loss function, we experi-
                                                         drops. Overall, the same peak appears for both
mentally set the weights a and b for the different
                                                         T EXT R ANK and S UMMER when λ1 ∈ [0.6, 0.7],
terms to 0.15 and 0.1, respectively.
                                                         which means that slightly higher importance is at-
   We used the Adam algorithm (Kingma and Ba,
                                                         tributed to the screenplay scenes that follow. In-
2014) for optimizing our networks. After experi-
                                                         tuitively, initial scenes of an episode tend to have
mentation, we chose an LSTM with 64 neurons for
                                                         high similarity with all other scenes in the screen-
encoding the scenes in the screenplay and another
                                                         play, and on their own are not very informative
identical one for contextualizing them. For the
                                                         (e.g., the crime, victim, and suspects are intro-
context interaction layer, the window l for comput-
                                                         duced but the perpetrator is not yet known). As
ing the surrounding context of a screenplay scene
                                                         a result, the undirected version of T EXT R ANK
was set to 20% of the screenplay length as pro-
                                                         tends to favor the first part of the story and the re-
posed in Papalampidi et al. (2019). Finally, we
                                                         sulting summary consists mainly of initial scenes.
also added a dropout of 0.2. For developing our
                                                         By adding extra importance to later scenes, we
models we used PyTorch (Paszke et al., 2017).
                                                         also encourage the selection of later events that
                                                         might be surprising (and hence have lower simi-
D   Additional Results
                                                         larity with other scenes) but more informative for
We illustrate in Figure 5 the performance (F1            the summary. Moreover, in S UMMER, where the
score) of the directed neural T EXT R ANK and            weights change in a systematic manner based on
S UMMER models in the unsupervised setting with          narrative structure, we also observe that scenes ap-
respect to different λ1 values. Higher λ1 values         pearing later in the screenplay are selected more
correspond to higher importance for the succeed-         often for inclusion in the summary.
ing scenes and respectively lower importance for            As described in detail in Section 3.3, we also

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infer the narrative structure of CSI episodes in the
supervised version of S UMMER via latent TP rep-
resentations. During experimentation (see Sec-
tion 5), we found that these TPs are highly corre-
lated with different aspects of a CSI summary. In
Figure 6 we visualize examples of identified TPs
on CSI episodes during test time alongside with
gold-standard aspect-based summary annotations.
Based on the examples, we empirically observe
that different TPs tend to capture different types
of information helpful for summarizing crime in-
vestigation stories (e.g., crime scene, victim, per-
petrator, motive).

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