A Computational Model of Narrative Generation for Suspense

A Computational Model of Narrative Generation for Suspense Yun-Gyung Cheong and R. Michael Young Liquid Narrative Group Department of Computer Science North Carolina State University Raleigh, NC 27695 [email protected], [email protected] Abstract that is leading to a significant outcome, b) intervening Although suspense contributes significantly to the events, and c) the outcome. enjoyment of a narrative by its readers, its role in dynamic Our approach attempts to manipulate the level of story generation systems has been largely ignored. This suspense experienced by a story’s reader by elaborating on paper presents Suspenser, a computational model of the story structure — making decisions regarding what narrative generation that takes as input a given story world story elements to tell and when to tell them — that can and constructs a narrative structure intended to evoke the influence the reader’s narrative comprehension process. desirable level of suspense from the reader at a specific To this end, we make use of a computational model of that moment in the story. Our system is based on the concepts comprehension process based on evidence from previous that a) the reader’s suspense level is affected by the number psychological studies (Brewer, 1996; Gerrig and Bernardo, of solutions available to the problems faced by a narrative’s protagonists, and b) story structure can influence the 1994; Comisky and Bryant, 1982). To generate suspenseful reader’s narrative comprehension process. We use the stories, we set out a basic approach built on a tripartite Longbow planning algorithm to approximate the reader’s model, adapted from narrative theory, that involves the planning-related reasoning in order to estimate the number following elements: the fabula, the sjuzhet, and the of anticipated solutions that the reader builds at a specific discourse (Rimmon-Kenan; 2002). A fabula is a story point in the story. This paper discusses our preliminary world that includes all the events, characters, and situations results and concludes with suggestions for further study. in a story. In our approach, the fabula is represented as a plan structure generated by Crossbow—a hierarchical, partial-order causal link planner based on the Longbow Introduction planning system (Young et al., 1994). A sjuzhet is a series Suspense contributes significantly to the enjoyment of a of events selected from the fabula and an ordering over narrative by its readers (Brewer and Lichtenstein, 1982; those events indicating the order in which they are to be Alwitt, 2002). Suspense is the feeling of excitement or presented to readers. The final layer, a discourse, can be anxiety that audience members feel when they are waiting thought of as the medium of presentation itself (e.g., text, for something to happen and are uncertain about a film). Although not directly discussed in this paper, significant outcome. discourse is important for the effective presentation of a This paper presents a computational model of suspense, story for the reader (Callaway and Lester, 2002). exploring the concept that a reader’s suspense level is We present Suspenser, a framework that constructs a affected by the number of solutions available to the narrative structure (i.e., sjuzhet) from a given story world problems faced by a narrative’s protagonists (Gerrig and (i.e., fabula) intended to evoke the given level of suspense Bernardo, 1994; Comisky and Bryant, 1982). In particular, (i.e., either high or low) from the reader. We assume in the our model focuses on plot-suspense, which differs from work described here that the stories we deal with all action-suspense in that the former is generated from plot contain conflict. For example, characters’ individual goals development and the latter is evoked by the reader may be negations of each others’, or the plans formed by observing physical action scenes (such as car chases in characters to achieve their goals may interfere with the film). For viewers to feel plot-suspense from a narrative, plans of other characters. While other dramatic devices the story should unfold in a specific manner. Brewer and such as the prolonging of resolution are also useful in Lichtenstein (1982) suggest that a suspenseful story should crating suspense, we focus here on suspense that arises as a be structured as a sequence involving a) an initiating event result of users’ consideration of these conflicts and their consequence on the protagonist’s goals. We expect that Suspenser can be incorporated into various narrative- centered applications such as games and interactive dramas Copyright © 2006, American Association for Artificial Intelligence in virtual worlds where the effect of suspense serves the (www.aaai.org). All rights reserved. user’s qualitative experience. Related Work In an interactive narrative system where the user participates as a character in the story, choices made by the Automated story generation has been extensively studied, user influence the story development. To provide the user with applications ranging from computer games to with an effective experience while she interacts with a education and training (Cavazza, 2002; Riedl and Young, virtual story world that the Oz system (Kelso et al., 1993) 2004; Mateas and Stern, 2003). However, little attention presents, Weyhrauch (1997) developed the Moe has been paid to computational story generation dealing architecture, which views the interaction between the user with suspense. As a result, this section reviews suspense and the system as a sort of an adversarial search. Moe related psychological research and interactive narrative consists of two main components: an evaluation function systems that focus on the user’s experience of the story. and an adversarial search. The evaluation function rates the Suspense is characterized by Vorderer (1996) as a quality of a sequence of user moves generated in the course phenomenon associated with three dimensions—type of of her experiencing the story world. The result of text, the reader, and the reader’s emotional process. The Weyhrauch’s experiment shows that the evaluation first dimension, type of text, suggests that the reader will function correctly approximates two human experts’ experience a higher level of suspense when reading text aesthetic evaluation on a user’s experience. In addition, the describing physical actions rather than when reading text article reports the effectiveness of the shallow searching describing a character’s thought and emotion. Vorderer method from the result of the experiments with simulated hypothesizes that the second factor, the readers, affects the users parameterized by confirmation to the system’s level of suspense that a text evokes based on their inter- guidance. And yet, the universal application of the shallow individual factors (e.g., age, gender) and intra-individual searching method is questionable. Nelson and Mateas factors (e.g., physical locations, moods). The last (2005) report that the Moe system has little impact on dimension, the reader’s emotional process, influences the improving the user’s experience in an interactive world experience of suspense: a reader will imagine a preferred based on an interactive mystery fiction Anchorhead. In the outcome for a character within a story that she identifies article, they ascribed the dissatisfactory result to the with, and consequently, if the preferred outcome looks shallow searching method that the Moe architecture unachievable, the reader feels suspense. employs. Previous psychological studies have explored story While Moe directs the user towards an ideal sequence of structure regarding suspense (Brewer, 1996; Brewer and moves, DEFACTO (Sgouros, 1999) determines the Lichtenstein, 1982). Brewer (1996) suggests the relevance subsequent event and its desirable outcome responding to of structural-affect theory, claiming that affective states in the user’s current choice. DEFACTO uses a rule-based plot the reader are provoked by arranging the temporal ordering manager to create story lines that conform to the concept of of the events underlying a story world. The theory explains plot of initial situation, a climax, a conflict and a resolution, that suspense could be evoked by presenting the events of a as first described by Aristotle. The plot manager builds a story chronologically to the reader while surprise and plot to provide dramatic experience for the user, who plays curiosity could be caused by hiding a critical fact or event the role of a protagonist in a story. The plot manager first early in the story world and disclosing it later in the text. constructs possible sequences of actions for each Film directors also arrange the filming of story elements to character’s goal and roles; its second stage selects action manipulate the reader’s beliefs for evoking suspense sequences that follow one the four dramatic patterns (i.e., (Gerrig, 1996). lifeline, rising-complication, reversal-of-fortune, and A number of AI researchers have presented user- irony); then it selects one dramatic situation that gives the centric views to story generation that are closely related to user a greater degree of participation as the next plot the approach we take here. Bailey (1999) suggests an element. These phases of generation and evaluation repeat approach to generating stories considered interesting by the sequentially until no new interesting interactions are found. reader by accessing the reader’s knowledge-base. His Finally, the resolution phase determines the outcome of system follows a cycle composed of four stages. The first each action, success or failure. Sgouros’ research proposes phase manipulates (i.e., generalize, specialize, detach, join) a user-centric view to story generation. Particularly, his the current reader’s knowledge-base to generate candidates evaluation phase conforms to the observations by Kelso et for the next story segment. The second measures the effect al (1993), who report that a user is strongly engaged when of each candidate story unit based on the reader’s she actively participates in the story even when the expectations and questions with the story-so-far.

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