Character's Psychology and Planning Formalisation

The psychology and feelings of characters is an essential part of most dramas and it is therefore surprising to see these under-represented in interactive narrative research, to the exception of simulation and educational storytelling. Another notable exception has been Mateas and Stern’s Façade system. One of the central aspects of Façade, which investigate the aesthetic satisfaction of the narrative.

In our own previous work (Cavazza, Charles, & Mead, 2002; Charles & Cavazza, 2004), we have developed an approach known as character-based storytelling, to the extent that characters’ roles, rather than a centralised plot model, serve as the main driver for narrative generation. This approach is suited to the generation of situations arising from the interaction between characters. However, we have come to realise the paradox that for a character-based approach, it is not more strongly based on the actual characters’ feelings and intentions, which form an integral part of narrative descriptions.

Hence, to this end we developed an IS system based on an adaptation of several chapters of a XIXth century French classic novel, Madame Bovary [Flaubert, 1856], which is well suited to a formalisation in terms of characters' feelings. Moreover, as part of the draft [Leclerc, Y. (1995). Plans et Scenarios de Madame Bovary. CNRS Edtions], Flaubert himself described the plot in the form of elementary plans and scenarios, together with an extensive description of the characters' psychology at various stages of the plot. This description is accurate enough to constitute the basis for an "ontology". These feelings range from traditional ones like boredom to fairly specific ones such as Irritated by vice or Emboldened by Love.

The resulting planning domain includes propositional representation of characters' in the form of:

• Physical atoms
• Elementary feelings qualified with a finite set of intensity values either:
  • Personal such as loneliness and embarrassment
  • Relative to other characters like affinity or concern
• Contextual flags which are symbolic predicate associated with a specific situation such as in_conversation.

In a similar fashion, we specified STRIPS planning operators as belonging to three categories depending on the type of modification they operate and the actions they trigger on the virtual stage:

• Physical operators determine for instance the characters navigation.
• Communication operators produce communicative actions which attempt to influence another character's feelings.
• Interpretation operators, which express more specific feelings and their consequences by decomposing them into more elementary ones.
Here an example of such operator, the interpretation Emboldened-by-Love where we can see that in the beginning the combination of Emma still being in love with her ex-lover and at the same time experiencing a new relationship will induce an increase in her self-confidence and a revival from a past relationship

This formalisation allowed us to use without any representational or algorithmic alteration, a heuristic search planning [Bonet, B., & Geffner, H. (1999). Planning as Heuristic Search: New Results. European Conference on Planning, ECP'99, pp. 360-372], controlling STRIPS-like operators to determine the next action to perform on virtual stage. In order to adapt it to real-time planning we have used RTA* [Korf, R. E. (1990). Real-time heuristic search. Artificial Intelligence, 42:2-3, pp. 189-211] as its underlying search algorithm, thus allowing backtracks in order to avoid dead-locks, and returning the best next action after a certain threshold time. Finally, by driving characters' behaviours, the evolution of characters' feelings throughout the story's unfolding constitutes a consistent narrative backbone .