IEEE TRANSACTIONS ON COMPUTATIONAL INTELLIGENCE AND AI IN GAMES, VOL. 7, NO. 1, MARCH 2015 39 Suspenser: A Story Generation System for Suspense Yun-Gyung Cheong, Member, IEEE, and R. Michael Young, Senior Member, IEEE Abstract—Interactive storytelling has been receiving a growing when they are anticipating the occurrence of some event and attention from AI and game communities and a number of compu- are uncertain about the event's outcome [13]–[15]. Our work tational approaches have shown promises in generating stories for focuses on the class of suspense associated with the perceived games. However, there has been little research on stories evoking specific cognitive and affective responses. The goal of the work likelihood of undesirable outcomes over preferred outcomes. we describe here is to develop a system that produces a narra- More specifically, implicit in much of the work we present here tive designed specifically to arouse suspense from its reader. Our is the notion articulated by GerrigandBernardo[16]inwhich approach attempts to create stories that manipulate the reader's they view a story's audience as active problem-solvers. In their suspense level by elaborating on the story structure that can influ- model of narrative comprehension, a reader's feeling of sus- ence the reader's narrative comprehension at a specific point in her pense is affected by the number of potential solutions she can reading. Adapting theories developed by cognitive psychologists, our approach uses a plan-based model of narrative comprehension anticipate for the dilemma faced by the protagonist. to determine the final content of the story in order to manipulate To generate suspenseful stories, we set out a basic approach the reader's suspense. In this paper, we describe our system im- built on a tripartite model, adapted from narrative theory, that plementation and empirical evaluations to test the efficacy of this involves the following elements: the fabula,thesjuzhet,andthe system. discourse [17]–[20]. A fabula is a story world that includes all Index Terms—Cognitive model, discourse planning, narrative the events, characters, and situations in a story. A sjuzhet is a generation, suspense. series of events selected from the fabula and an ordering over those events indicating the order in which they are to be pre- sented to readers. In our approach, the fabula and sjuzhet are I. INTRODUCTION represented as plan structures. The final layer, the discourse, ARRATIVE has long been associated with games. While can be thought of as the medium of presentation itself (e.g., N most games employ linear narrative as a back story only text, film). In this paper, we present Suspenser, a framework that to engage the gamer with gameplay, classical adventure games determines narrative contents (i.e., sjuzhet) from a given story and some recent action games such as Heavy Rain [1], L.A. world (i.e., fabula) intended to evoke suspense on the part of the Noire [2], Bioshock Infinite [3], and Fallout: New Vegas [4] reader. The rest of this paper is organized as follows. Section II are designed to offer the player with high narrative experience reviews relevant work in narrative psychology and AI. Section as well. A number of story generation systems and AI tech- III describes our tripartite model for story generation. Section niques have been developed for creating highly interactive sto- IV details the Suspenser framework, followed by our evalua- ries [5]–[9]. And yet, there has been little attention to affective tion that assesses the performance of Suspenser in Section V. properties of narrative, which are, in fact, fundamental to its ap- The last chapter concludes with a discussion of the limitations preciation by the user. The feeling of suspense, surprise, and of our system and the plan for future work. curiosity is generally expectedinreadingandviewingnarrative forms [10]–[12]. Among these emotions, suspense has drawn much attention from psychologists due to its significant impact II. RELATED WORK on the reader's enjoyment. In the empirical study conducted by Theories in psychology and cognitive science view the sus- Brewer and Lichtenstein [10], the participants reported that sus- pense in the reader's mind as related to diverse narrative el- pense is cardinal for discerningastoryfromamereseriesof ements. The reader's disposition toward protagonists or out- events and expressed high satisfaction about their narrative ex- comes is a primary condition for her to feel suspense [13], [21]; periences when suspenseful events were presented in the stories. the reader should be able to anticipate the potential for con- Our approach addresses the problem of creating suspense in flicting outcomes of an event [12]. The reader's suspense would narrative, which keeps the user engaged in the various plots, increase when she perceives likelihood of undesirable outcomes giving them high entertainment value. We view suspense as the relative to preferred outcomes [14], [15], [22]–[26]. Prolonging feeling of excitement or anxiety that audience members feel the length of time the reader anticipates an event or outcome that is potentially harmful to a protagonist can also heighten her sus- Manuscript received July 10, 2013; revised February 03, 2014; accepted May pense [26]. Discrepancies between the knowledge of the story 06, 2014. Date of publication May 14, 2014; date of current version March 13, world held by characters and that held by the reader can be 2015. This work was supported in part by Award 0092586 and Award 0414722, and the EU FP7 ICT Project SIREN (Project: 258453). used as a device to induce the reader's suspense [11], [12], [23]. Y.-G. Cheong is with the College of ICE, Sungkyunkwan University, South In the well-known emotion model OCC [13], suspense could Korea (e-mail: [email protected]). be regarded as a specific type of prospect-based emotion,one R. M. Young is with the Department of Computer Science, North Carolina State University, Raleigh, NC 27695 USA (e-mail: [email protected]). which is evoked when an individual anticipates the occurrence Digital Object Identifier 10.1109/TCIAIG.2014.2323894 of events whose outcome is uncertain. Their account of suspense 1943-068X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 40 IEEE TRANSACTIONS ON COMPUTATIONAL INTELLIGENCE AND AI IN GAMES, VOL. 7, NO. 1, MARCH 2015 involves hope and fear; a person hopes his favorable conse- quence will be realized while he fears the occurrence of undesir- able consequences. Gerrig and Bernardo [16] hypothesize that a reader's feeling of suspense is affected by the number of poten- tial solutions available for the protagonist. Under this model, an audience will feel an increased measure of suspense as the number of options for the protagonist's successful outcome(s) decreases. To confirm this hypothesis, Gerrig and Bernardo per- formed a number of experiments with human subjects. The sub- jects were provided different text versions of a story where a protagonist is in danger and tries to escape. The various ver- sions of the story differed in the number of solutions available to the protagonist. After reading the text, subjects were asked to rate their estimation of the likelihood of the protagonist's escape as well as their suspense levels. The data from the experiments showed that the readers reported higher suspense as possible so- lutions were eliminated. Quite a number of interactive storytelling systems have Fig. 1. Example fabula plan structure, illustrating a father getting a toy for his seven-year old son Ben as a Christmas gift. In the diagram, temporal ordering been implemented so far [5]–[9], [27]–[31]. In particular, constraints ( ) proceeds from the top to the bottom. Rectangles represent a MINSTREL [31], a case-based approach to modeling human series of plan steps ( ), with each action's preconditions enumerated above its creativity, is most comparable to our approach. At the core rectangle. An arrow between two actions indicates a causal relationship that holds between the two ( ), meaning that the effects of the action at the starting of the MINSTREL design is a transform-recall-adapt process. point of the arrow establishes a precondition for the action at the arrow's end On receiving a problem specification as input, MINSTREL point. Given an initial state (i.e., the father is poor and he has a ring), this plan retrieves a case from its memory that is similar to the problem is constructed automatically to achieve the goal (has Ben toy). The condition of S3:Move action is true under the closed-world assumption: given as input. If the case is identical to the problem, the a condition not specified in the initial state is assumed to be false. The plan can original solution is used. If the case differs from the input be described in text as: “A poor father traded his wedding ring for the toy that problem, the original solution is modified. As an end-to-end his son Ben wants to have. He then put the toy under their Christmas tree. The next day Ben walked to the tree and found the toy that his father left.” system, MINSTREL extensively attempts to solve multiple story-generation issues such as themes, coherency, characteri- zation, tragedy, suspense, and foreshadowing. In MINSTREL suspense is created by fabula-level generation, relying on the course generator. The fabula is represented as a plan data struc- psychological evidence that readers feel more suspense when ture, as in Definition 1, created in response to a story request they strongly care about the character [13], [21] and when specifying the initial and goal states of the story world. To gen- the presentation of a significant outcome is prolonged [26].