Abstract. To inform the design of interactive drama systems, we investigate the experience of an interactor being part of a story that they can have a fundamental influence on. Improvisational theatre might serve as a model for this experience, where there is no pre-scripted plot; each of its actors shares responsibility for the collaborative emergence of a story. This requires a performer attitude from the interactor. We describe an experiment in which improv actors create a story together with subjects who have no improv experience, to find out how we can characterize this experience, and how it might be achieved. Our results support a recent hypothesis that an interactor in interactive drama might be treated as a collaborative performer rather than an (antagonistic) player. Key words: Interactive Storytelling, Dramatic Presence, Interactive Drama, Improvisational Theatre