Dialogue Generation in Character-based Interactive Storytelling

In recent years, there has been significant progress in developing Interactive Storytelling systems, in particular in terms of the underlying AI techniques. There has been an emerging consensus on the AI approach, in particular the use of planning systems. However, these have concentrated on the generation of a sequence of narrative actions, staged through the behaviour of virtual actors. In order to achieve the long-term objective of implementing interactive media that would reproduce the aesthetic qualities of traditional films, most interactive storytelling systems are still missing the ability to generate dialogues between characters. We describe an extension of our Interactive Storytelling approach which integrates dialogue generation within narrative situations. Our unit of generation is the dialogue act (pair of utterances) and one main objective is to reproduce realistic dialogue phenomena based on implicit forms of expression. The emphasis is also on adopting a unified approach relating narrative representations to the linguistic form of the generated utterances. From a representation of active narrative goals and the emotional affinity between characters, our system is able to derive a set of parameters governing various aspects of the linguistic form to be generated. We illustrate this approach on several implemented examples. Introduction and Objectives Dialogues between feature characters are an essential component of traditional media, such as films and plays. In that respect, interactive narratives featuring virtual humans as actors have not yet fully addressed this aspect. Most work in interactive narratives remains centred on the generation of meaningful actions staged as 3D animations. While a certain consensus is emerging on the types of techniques (Cavazza et al. 2002) (Riedl and Young 2004) (Young 2004), which can produce sequences of narrative actions, it is important to investigate how dialogue can be generated as part of these actions. This is a natural extension of the narrative description, as many actions actually consist in communicative actions, as well as an essential aspect of the actual staging of the generated Copyright © 2005, American Association for Artificial Intelligence (www.aaai.org). All rights reserved. narrative considering the endeavour of obtaining film-like media. Interactive Storytelling (IS) techniques should be made compatible with the requirements of dialogue generation to support some of the aesthetic components of traditional media (intended as including humour, witticisms, etc., so characteristic of real-world media productions such as plays, dramas, sitcoms, cartoons ...). While recognising that this constitutes a formidable challenge, in this paper we describe first steps for the principled integration of dialogues into plan-based interactive storytelling. Taking as a starting point our approach for the generation of virtual narratives in the sitcom genre, we investigate how narrative formalisms can be properly related to linguistic structures supporting dialogue generation. IS can be described as comprising two levels. The first level is dedicated to the generation of narrative actions, some of which will consist of communicative actions between characters. The second level corresponds to the proper dramatisation of narrative actions. Physical actions and situations are staged essentially through camera placement. Communicative actions, on the other hand, are dramatised by the generation of appropriate dialogues. The generation of dialogue lines should be appropriate to the narrative genre considered, in particular in terms of linguistic phenomena. Our main objective is to integrate dialogue generation within our Interactive Storytelling technology (Cavazza et al., 2002) in a way which should be consistent with the story genre and the overall dramatisation of narrative actions. In particular, the dialogue’s linguistic contents should be able to reflect the emotional relations between the various characters in terms of style or use of dialogue moves. Relation to Previous Work Several researchers in IS have tackled the problem of dialogue, mostly between user and virtual actors. One of the most advanced systems in this regard is undoubtedly Façade (Mateas and Stern 2002) (Mateas and Stern 2004), a first-person IS system in which the user intervenes into a dispute between two characters through natural language input. Façade is one of the early attempts at relating narrative functions to dialogue acts in a principled fashion. This is achieved through the fine-grained definition of dialogue acts some directly addressing narrative functions (such as “flirt”, “pacify” or “advise”), while others correspond to generic communication functions (Mateas and Stern 2004). However the dialogue in Façade is more concerned with individual replies and dialogue progression than with narrative elements arising from the dialogue itself (such as witticisms, etc.). The latter aspect has been the major focus of our previous research (Charles et al. 2004) on the understanding of dialogue jousts in “James Bond” narratives. Another approach taken in the MRE system consists in integrating strongly dialogue and narrative by resorting to a task-based dialogue (Traum et al. 2003), such a negotiation dialogue which naturally supports the mode of communication to be expected in taking decisions under critical conditions. Figure 1: Characters’ roles and the influence of their affinities towards other characters. Context and Application Our work on IS has been essentially based on the sitcom genre, which provides a manageable context for narrative generation. In addition, the importance of the notion of situation provides a framework for assessing the validity of the technical approach, as well as for early attempts at evaluating scalability (Charles and Cavazza 2004). We defined our approach as character-based interactive storytelling (Cavazza et al. 2002) to emphasise the specific stance it takes towards the duality of character and plot. The baseline plot is described through a set of independent roles for each character, which are formalised as HTN plans. During system operation, each character acts independently following its own plan. The dynamic interaction between characters and their competing for action resources (including other characters’ availability) generates narrative situations whose sequence produces the story. One central aspect of the system, where dramatisation and generation mechanisms converge, corresponds to characters’ actions failure. These are dramatised to constitute narrative elements, and at the same time lead to re-planning which is the basis for story generation and variability. This re-planning process also supports user intervention, as it influences the emerging story mostly through failure of characters’ actions. This results in a combinatorics of plot elements, where a set of basic role definitions can generate several hundred different narratives. Another advantage of the character-based approach is its ability to integrate various relevant representations around the plan-based representation of a character’s role. In particular, it can include specific emotional representations describing the affinities between characters, which are often central to that narrative genre (Figure 1). Affinities between characters play a central role in selecting contextual response to a situation (for instance, joining or interrupting a conversation between characters). As we shall see, they will play an important role in the generation of dialogues between characters as well. IS systems rely on a cast of virtual actors whose actions constitute the narrative. For many reasons, including historical ones, IS technology has developed by staging physical actions first, with very few exceptions (Mateas and Stern 2002). However, even when taking an overall physical stance, key narrative events are often better staged through dialogue. Relations between Narrative and Dialogue The interaction between characters constitutes most of the narrative action and as such should be not only meaningful, but should be staged with the same aesthetic preoccupations which characterise the sequencing of narrative actions to constitute a story. This means that the dialogue itself should be staged through a choice of linguistic expressions which should display the properties of a real dramatic dialogue (Figure 2). Traditional narratology works at a level of description which stands above that of linguistic expression. It was Bremond (Bremond 1973) who first thoroughly investigated the relations between narrative roles and their translation in terms of linguistic expression as part of intercharacter dialogue. This was a consequence of Bremond’s narrative theory, which is centred on a classification of characters’ roles which often hints at communicative aspects, as with the influencer role. Bremond proposed to map the narrative role onto rhetoric functions which will constitute the linguistic embodiment, and at the same time the dramatisation of a narrative action whose nature is communicative. This can be briefly illustrated by considering Bremond’s role of influencer (which is selfexplanatory). A character wishing to influence another will use different rhetoric functions, such as advice, threats, seduction, for expressing that influence, depending on his understanding of the recipient’s goals (Bremond 1970). To some extent, Bremond’s proposal was the first one to attempt to properly map narrative functions to communicative acts. However, it did not address the conversational aspects of the characters’ dialogue, nor did it specify the semantic content of corresponding utterances. Figure 2: An invitation to a party. In the next section, we propose an approach inspired from Bremond’s model, but adapted to a computational approach. Firstly, we will use affinities between characters in the narra