Evaluating synthetic actors Sandy Louchart and Ruth Aylett 1 Abstract 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. We present the evaluation of this approach and some initial results on whether it produces more ‘interesting’ narratives. 1 - INTRODUCTION Narrative has become a topic of great interest in video and computer games development as a way of drawing the player into the game play [16], and is seen as a focus for the development of mobile and Augmented Reality-based gaming [21]. Much active research addresses the generic use of interactive graphical environments and intelligent synthetic characters to extend the power of narrative in new ways [16]. Specifically it has played a central role in a number of interactive graphics-based e-learning systems both for adults [24] and children [9, 18]. Narrative is also used as a generic method for adding intelligence to virtual environments, for example, through the development of virtual guides [4]. 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 [13]. 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 [19]. 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 characters autonomously select their actions in response to their sensing of the state of the virtual world – strong autonomy [15]. Merging the roles of spectator and author evades rather than reconciles the contradiction since authoring merely allows a plot-based approach to be maintained; this approach has been exploited in a number of systems [9, 18, 20]. The God-like perspective of games such as ‘The Sims’ gives the privileged user overall responsibility for the activity within the virtual world in a similar fashion. Creating a branching narrative is another solution [24, 14], though either the user is constrained into a few key choices, breaking their immersion in the narrative world, or characters must be supplied with “universal plans” [23] covering every possible response to whatever the user does. Façade [15] is an impressive example of the result of doing this, using the concept of ‘beats’, based on an adaptation of Aristotelian theory, but required substantial authoring effort for a short (20 minute) narrative, with clear implications for scalability. Limiting the interactive stance of the user is a third 1 MACS Heriot-Watt University EH14 4 AS e-mail: {sandy, ruth}@macs.hw.ac.uk solution: one may apply concepts such as Boal’s [3] spect- actors, in which participation and spectating are episodically interleaved [2]. In [5] characters have universal plans expressed as AND/OR trees but the role of the user is confined to manipulation of key objects, forcing character re-planning. Strong autonomy for characters offers a potential solution to the problem of interactivity 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, it seems possible that interaction between strongly autonomous characters could under 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. However, an existential proof of the EN approach can be found in interactive forms such as improvisational drama and human RPGs: in the former actors start from a well-defined initial state and strong roles and select ‘dramatically-interesting’ actions, while in the second, a game-master dynamically manages the experience of the autonomous participants [13]. In this work we discuss the application of both these ideas within the additional framework of affective appraisal theory. The hypothesis being 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 [22] 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 [11]. 2 – NARRATIVE AND EMOTION 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 [7] have integrated affect into their agent architectures [8, 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) [17], used by the FatiMA agent architecture which formed the basis for the work described here. The OCC model is an approach based on a valenced (good or bad) reaction to an event and the structure of emotions it defines can be seen as a hierarchical taxonomy organising 22 emotion types.