The Virtual Storyteller is a multi-agent framework for automatic story generation. In this paper we describe how plots emerge from the actions of semi-autonomous character agents, focusing on the influence of the characters’ emotions on plot development

Anton Nijholt

Dirk Heylen

Er Rensen

Mariet Theune

Rieks Op Den Akker

Sander Rensen

English

Theune, Mariët

Rensen, Sander

op den Akker, Rieks

Heylen, Dirk

Nijholt, Anton

University of Twente Research Information

Emotional Characters for Automatic PlotCreationM. Theune, S. Rensen, R. op den Akker, D. Heylen, and A. NijholtUniversity of Twente, PO Box 217, 7500 AE Enschede, The Netherlands{theune|rensen|infrieks|heylen|anijholt}@cs.utwente.nlAbstract. The Virtual Storyteller is a multi-agent framework for au-tomatic story generation. In this paper we describe how plots emergefrom the actions of semi-autonomous character agents, focusing on theinfluence of the characters’ emotions on plot development.1 IntroductionThe Virtual Storyteller is a multi-agent framework for automatic story genera-tion. Its functionality includes plot creation, narration and presentation. Plotsare automatically created by semi-autonomous character agents that carry outdifferent actions to achieve their goals. To guarantee that the resulting plots willbe well-structured, the characters are guided by a director agent, who provides(some of) them with specific goals, and who can forbid their actions if theydo not fit into the general plot structure. The resulting plot is converted intoa natural language text by a narrator agent, and presented to the user by anembodied, speaking presentation agent. The global architecture of the VirtualStoryteller is shown in Fig. 1.Fig. 1. Architecture of the Virtual StorytellerWith the first version of the Virtual Storyteller [12] we aimed at meeting twobasic plot requirements: consistency at the level of character actions (throughthe use of goal-oriented characters) and well-structuredness (through the use ofa director agent). Another storytelling system that combines a character-basedappproach with the use of a virtual director is Teatrix, which is aimed at aidingstory creation by children [3]. In the Oz project [4], a ‘drama manager’ was usedto ensure the occurrence of some essential plot points during interactive storycreation. The ACONF system [8] uses a different approach: here, a coherent plotstructure is achieved by representing the plot as a partial-order plan, which isconstructed by a group of expert systems representing the story characters.Consistency and well-structuredness are essential for achieving a well-formedplot, but do not guarantee that the resulting story will be entertaining. For that,a plot should have additional properties such as unexpectedness, suspense andthe presence of believable characters. A common approach to make characteragents believable is to provide them with emotions. Most computational mod-els of personality and emotion are based on Ortony, Clore and Collins’ eventappraisal theory, known as the OCC-model [6]. Examples are Elliott’s AffectiveReasoner [1] and Neal Reilly’s Em system [5]. A simplified version of Em wasincorporated in Loyall’s Hap architecture for believable agents [2], used in theOz project. This work was the main source of inspiration for our recent extensionof the Virtual Storyteller with an emotion model for the characters.In the following sections, first we briefly sketch the emotion model used inthe current version of the Virtual Storyteller. Then we discuss the influence ofthe character’s emotions on plot creation, showing how the characters’ abilityto adopt goals that are based on their emotions, rather than dictated by thegeneral storyline, increases their believability and may give rise to unexpected(but nevertheless consistent) plot twists. We illustrate this with an example.2 Emotions and ActionsIn the Virtual Storyteller, the emotional state of an agent is represented by pairsof corresponding positive and negative emotions and their intensities. Emotionsthat are directed at the agent itself are hope-fear, joy-distress, and pride-shame.Emotions directed at other agents include admiration-reproach, hope-fear, andlove-hate. Each emotion pair has an intensity value, represented by a naturalnumber on a scale from -100 to 100. For example, a value of 20 for hope-fearmeans that the character experiences hope with an intensity of 20 (and no fear);a value of -20 means exactly the opposite.Following the OCC-model [6], the emotional state of an agent changes inreaction to events, actions and objects. If an agent comes close to achievingits goal, it experiences hope, and if its current goal proves to be unachievable,the agent experiences distress. If an agent performs an action meeting its ownstandards (which are part of its personality model), the agent experiences pride.If an agent meets another agent which it likes (also part of the personalitymodel), it experiences joy. The intensity and duration of these emotional effectsdepend on the personality parameters of the agent, which can be changed by theuser. For example, some characters may be frightened more easily than others,or may forget their anger more quickly.Based on its emotional state, a character may develop certain action ten-dencies such as friendly or unfriendly, aggressive or passive behaviour (cf. themapping of emotions to action styles in Hap [2]). The action tendencies in theirturn influence the importance a character attaches to certain goals. The relationbetween the emotions and the action tendencies of a specific character is laiddown in a number of action tendency scripts, which are part of the character’spersonality model. For instance, strong fear could lead to passive behaviour inone character (increasing the importance of the goal to flee) and to aggressivebehaviour in another (increasing the importance of attacking the object of fear).3 Plot Creation Using EpisodesThe Virtual Storyteller is aimed at the generation of short stories in the fairy taledomain. An advantage of this domain is that most fairytales have a common,fairly simple plot structure. The stories generated by the Virtual Storytellerconsist of four global episodes, which are loosely based on Greimas’ reformulationof Propp’s story functions [7]:1. initial state of equilibrium (a description of the initial setting of the story)2. disruption of the state of equilibrium (something bad happens)3. mission of the hero (the hero tries to restore the state of equilibrium)4. return to state of equilibrium (or not, if the hero fails)At the start of the story generation process, the director lays the basis for eachof the episodes by selecting four ‘episodic scripts’ from a database. An episodicscript contains information about the setting, goals and constraints of an episode.The setting specifies which locations, objects and characters are present in theepisode. The episodic goals are goals which must be reached within the episodeby (some of) the characters. Typically, episodic scripts of type 2 (disruption) willinvolve a villainous character with the episodic goal of performing some evil deed,and episodic scripts of type 3 (mission of the hero) will involve a character withthe episodic goal of putting things right again. Finally, the episodic constraintsspecify certain actions that are not allowed in the episode. For example, it mightbe disallowed to kill a certain character before its episodic goal is reached. Thesethree episode properties define the limits within which the character agents areallowed to act while constructing the episode.After the episodic scripts have been selected, a story world is created accord-ing to the specified setting, and the character agents are initialized. The actualepisode is then created by the characters carrying out actions to reach theirindividual goals. These goals can be the episodic goals that have been assignedto them by the director, but they can also be goals that were spontaneouslyadopted, based on the character’s emotions (see section 4). Before a charactercan carry out a planned action, it must ask the director for permission. Thedirector checks if the action violates any episodic constraints, or any global con-traints (for instance, the director might forbid numerous repetitions of an actionbecause they would make the story boring). If no constraints are violated, thecharacter is allowed to carry on with its action, and the story world is updatedaccordingly. When one of the characters reaches its episodic goal, the episodeends. Because the characters can also adopt goals other than the episodic goal,there are many different ways in which an episodic script can develop beforean episodic goal is reached. This variation can be further increased by changingthe personality parameters of the characters (see section 2). Still, all variationswill respect the global plot structure, because of the fixed order of episodes incombination with the local episodic goals and constraints.In our current set-up, the episodic scripts are created and combined by ahuman author.1 In terms of the ‘scalable autonomy’ approach of Spierling etal. [9], this means that there is no autonomy at the story level: the global plotstructure is predefined. However, there is high autonomy at the scene (episode)and character levels: the actions carried out within an episode are not specifiedin advance, and the characters can make their own decisions (guided by thedirector, which is itself an autonomous agent).4 Goal Selection: Emotions Taking OverIn the Virtual Storyteller, each character agent has a fixed set of potential goals.The importance that a character attaches to these goals varies depending on itsaction tendencies (see section 2). In addition, one of its goals may have the statusof ‘episodic goal’, in which case it has a very high importance (90 out of 100)throughout the episode. Instead of having a character simply adopt the goal withthe highest importance, we use a probabilistic approach to goal selection, wherea goal’s importance corresponds to the probability that it will be adopted. Inother words, an episodic goal has a 90% chance of being selected for pursuit, butsome ‘emotional’ goals may also have a high probability, due to the character’semotion-induced action tendencies. This means that a cowardly hero with theepisodic goal of killing the villain could be equally likely to adopt the goal ofrunning away from the villain because of its fear.As we will illustrate with some examples below, probabilistic goal selectionleads to variation and unexpectedness in the generated stories. However, thereis risk of ‘overdoing’ it by having too much random switching between goals.Therefore, if a character has already carried out the first step of a plan towardsachieving some goal, that goal will be maintained until it is reached (or madeunreachable by changes in the story world). The goal is only abandoned if thecharacter’s emotions run so high that they cause the importance of some othergoal (e.g., fleeing from an enemy) to exceed a pre-specified high value. Anotherrisk of our approach is that a character’s sudden change in behaviour, due tothe adoption of a new goal, may seem incomprehensible to the user [10]. In the1 As our script database grows, it will become more important to have a proper,automatic selection mechanism that only combines compatible scripts. Also, in thefuture we would like to distinguish more than four types of episodes, by includingmore of Propp’s functions. The sophisticated, real-time scene selection mechanismof the GEIST system [9] could serve as an example here.Virtual Storyteller, we can solve this by having the narrator explicitly describethe character’s motivation. Currently, this is only done to a very limited extent,and further work on this is required.5 An ExampleHere we illustrate the plot creation process by describing the different realisationsof a simple script of type 2(disruption). The script involves two characters, aprincess (Diana) and a villain (Brutus), who hate each other. Diana fears Brutus,but Brutus does not fear her. Diana has the episodic goal to kill Brutus, andBrutus has the episodic goal to capture Diana. Initally, the characters are indifferent locations. In yet another location, a sword can be found. We first discusswhat happens given the default, rather stereotypical action tendency scripts ofthe princess and the villain. These scripts specify (among other things) thatDiana is likely to behave passively when experiencing emotions such as fear andhate, whereas these same emotions trigger aggressive behaviour in Brutus.In most episodes that were generated on the basis of this script, the charactersimmediately start pursuing their episodic goals: Brutus starts walking towardsDiana to capture her, and she sets out to get the sword, needed to kill Brutus.Occasionally, one of the characters adopts another, emotionally motivated goalinstead, such as singing out of joy. This occurs rarely, however, because initiallythe character’s emotions have low intensity, so the importance of the associatedgoals is also relatively low. Whatever goal they start out with, the two characterseventually meet each other. When meeting Brutus, Diana is usually overcome byfear, causing her to abandon her episodic goal (even if she has already found thesword) and to run away instead. Brutus, who will act aggressively even if Dianahas the sword, will follow her, either to capture or to attack her (the latter goalbeing motivated by his aggression). In the end, Diana is inevitably cornered byBrutus without any possibility to flee. If she has the sword, she is very likelyto use it now, since killing Brutus is now her highest priority goal that can beattained. Otherwise, she is most likely to scream, because this is the only passivebehaviour left open to her. If she does not have the sword, Diana may also gofor the less likely goal of attacking Brutus by hitting or kicking him, but Brutusis stronger and will eventually capture her anyway.We have also tested what happens, given the same episode script, if the actiontendency scripts of the two characters are exchanged. In that case, the divisionof power is more equal, because although Brutus may be stronger than Diana,he is now more fearful and passive than she is. This leads to longer episodesinvolving more chase sequences and mutual attacks. In the end, as in the firstversion, the outcome depends on whether Diana manages to get to the swordin time or not. All in all, a large variety of episodes can be generated basedon the same script. The probabilistic goal selection may give rise to unexpectedcharacter actions, but these actions can always be traced back to the character’semotions. Properly expressing these inner motivations in the narration of theplot will be our next challenge.6 Conclusions and Future WorkThe version of the Virtual Storyteller we have described here has been fullyimplemented, albeit with a limited set of rules and episodes. However, our ex-periences so far indicate that even with this limited implementation, a largenumber of different stories can be created. Further extension of the system withmore possible goals, actions and objects is needed to show the full potential ofour approach. As pointed out above, another important area of further work isthe conversion of the plot into natural language, so that the underlying moti-vations of the characters are properly put across. With respect to presentation,we are currently working on the automatic generation of ‘storytelling prosody’and on the conversion of abstract plots to 2D animations, as an alternative pre-sentation mode for the Virtual Storyteller. Finally, in the longer run we wantto extend the Virtual Storyteller so that the emotions of the user can be takeninto account too, by learning rules to predict the effect on the user of differentcharacter actions. These rules can then be used to guide the decisions of thedirector (cf. the narrative sequencer employed in IDtension [11]).References1. C. Elliott. The Affective Reasoner: A Process Model of Emotions in a Multi-AgentSystem., Ph.D. thesis, Northwestern University, 1992.2. B. Loyall. Believable Agents: Building Interactive Personalities. Ph.D. thesis CMU-CS-97-123, Carnegie Mellon University, 1997.3. I. Machado, R. Prada, and A. Paiva. Bringing drama into a virtual stage. Proceed-ings of the Third International Conference on Collaborative Virtual Environments(CVE 2000), ACM Press, 2000.4. M. Mateas. An Oz-Centric Review of Interactive Drama and Believable Agents.Technical Report CMU-CS-97-156, Carnegie Mellon University, 1997.5. S. Neal Reilly. Believable Social and Emotional Agents, Ph.D. thesis CMU-CS-96-138, Carnegie Mellon University, 1996.6. A. Ortony, G.L. Clore and A. Collins. The Cognitive Structure of Emotions. Cam-bridge University Press, 1988.7. V. Propp. Morphology of the Folktale. University of Texas Press, 1968.8. M. Riedl and R.M. Young. Character-focused narrative generation for execution invirtual worlds. Proceedings of the International Conference on Virtual Storytelling,47-56, 2003.9. U. Spierling, D. Grasbon, N. Braun and I. Iurgel. Setting the scene: Playing digitaldirector in interactive storytelling and creation. Computers & Graphics 26, 31-44,2002.10. P. Sengers. Designing comprehensible agents. Proceedings of the 16th InternationalJoint Conference of Artificial Intelligence (IJCAI), 1227-1232, 1999.11. N. Szilas. IDTension: A narrative engine for interactive drama. Proceedings of theTechnologies for Interactive Digital Storytelling and Entertainment (TIDSE) Con-ference, 187-203, 2003.12. M. Theune, S. Faas, A. Nijholt and D. Heylen. The Virtual Storyteller: Storycreation by intelligent agents. Proceedings of the Technologies for Interactive DigitalStorytelling and Entertainment (TIDSE) Conference, 204-215, 2003.

Emotional characters for automatic plot creation

An Oz-Centric Review of Interactive Drama and Believable Agents.

Believable Agents: Building Interactive Personalities.

Believable Social and Emotional Agents,

Bringing drama into a virtual stage.

Character-focused narrative generation for execution in virtual worlds.

Designing comprehensible agents.

IDTension: A narrative engine for interactive drama.

Morphology of the Folktale.

Setting the scene: Playing digital director in interactive storytelling and creation.

The Aﬀective Reasoner: A Process Model of Emotions in a Multi-Agent System.,

The Cognitive Structure of Emotions.

The Virtual Storyteller: Story creation by intelligent agents.

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.10.1629

Emotional characters for automatic plot creation

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