In this paper, the authors describe a method for implementing the behaviour of artificial actors in the context of interactive storytelling. They have developed a fully implemented prototype based on the Unreal Tournament™ game engine, and carried experiments with a simple sitcom-like scenario. The authors discuss the central role of artificial actors in interactive storytelling and how real-time generation of their behaviour participates in the creation of a dynamic storyline. They follow previous work describing the behaviour of artificial actors through AI planning formalisms, and adapt it to the context of narrative representation. In this context, the narrative equivalent of a character's behaviour consists of its role. The set of possible roles for a given actor is represented as a hierarchical task network (HTN). The system uses HTN planning to dynamically generate the character roles, by interleaving planning and execution, which supports dynamic interaction between actors, as well as user intervention in the unfolding plot. Finally, the authors present several examples of short plots and situations generated by the system from the dynamic interaction of artificial actors.