From Synthetic Characters to Virtual Actors

We discuss the extension of an emotionally-driven agent architecture already applied to the creation of emergent narratives. Synthetic characters are enhanced to perform as actors by carrying out a second cognitive appraisal, based on the OCC model, of the emotional impact of their projected actions before execution. Introduction – Narrative and Emotion Narrative has become a topic of great interest in video and computer games development as a way of drawing the player into the gameplay [12], and is seen as a focus for the development of mobile and Augmented Reality-based gaming [16]. Much active research addresses the generic use of interactive graphical environments and intelligent synthetic characters to extend the power of narrative in new ways [12]. Specifically it has played a central role in a number of interactive graphics-based e-learning systems both for adults [19] and children [6, 14]. The key characteristic of all these environments is interactivity: users expect to move freely and interact at will with objects and synthetic characters. Yet this interactional freedom clashes badly with the conventional narrative requirement for a definite structure, creating a narrative paradox [9]. A plot-based narrative structure supposes the right actions at the right places and times but these may not be those the user chooses to carry out [15]. More generally, an authorial plot-based view of narrative where particular actions must execute in a particular order conflicts with a character-based view where strongly autonomous [11]. Characters select their actions in response to their sensing of the state of the virtual world. Strong autonomy for characters offers a potential solution to the narrative paradox since if synthetic characters are allowed to autonomously select actions, then a participating user can also be allowed to do so on the same terms. Given that in general, structure can emerge from interaction between simpler elements, we conjecture that interaction between strongly autonomous characters can under Copyright © 2007, Association for the Advancement of Artificial Intelligence (www.aaai.org). All rights reserved. specific circumstances produce narrative structure, or an emergent narrative (EN) [1]. The main objection to character-based narrative based on strong autonomy is that there is no guarantee that interesting narrative structure will result precisely because characters are responding to their internal state and individual goals in choosing actions and not to the overall story structure. This approach however prioritises the actions and experience of the participating user/player rather than the overall drama as perceived from a spectator/observor point of view. It focuses on the dramatic experience of the user [9, 18]. The specific hypothesis explored is that an autonomous agent that explicitly assesses the emotional impact of its actions on other agents around it, much as an actor would, will produce a more engaging emergent narrative than one that only uses its own ‘in-role’ emotional state to select its next action. Other virtual actors [17] have not tried to assess the differential emotional impact of a set of possible ‘in-role’ actions, making this a novel approach. Because it uses emotional impact, it is also different from assessing the goals or plans of other agents [7]. If narrative is to emerge from interaction between characters, then the character architecture is fundamentally important. It is the contextual relevance and richness of the actions selected by each character that will or will not produce sequences with the post-hoc structure of a story: that is a coherent compound of external interest and surprise (causal chains of actions) with internal perceived intentionality and emotional impact (motivation and expressive behaviour). Displaying role-specific emotional reactions to the actions of other characters and the emotion behind their own actions is an important component of successful human acting. For this reason a number of researchers in synthetic characters, starting with Elliot’s Affective Reasoner [4] have integrated affect into their agent architectures [5, 2], usually drawing on cognitive appraisal theory. Appraisal is the human perceptual process through which objects, other characters and events are related to the needs and goals of an individual, generating a resulting emotional response and thus linking emotion to cognition. The most widely implemented system is the taxonomy of Ortony, Clore and Collins (OCC) [13], used by the FatiMA agent architecture