An emergent narrative is a narrative that is dynamically created through the interactions of autonomous intelligent virtual agents and the user. Authoring in such a system means programming characters rather than defining plot and can be a technically and conceptually challenging task. We are currently implementing a tool that helps the author in this task by training the characters through demonstration of example story lines (rehearsals), rather then explicit programming. In this paper we argue that this tool is best used by a group of authors, each providing an example story and that in order to achieve true emergence, collective authoring is required. We compare the rehearsal based authoring method of our authoring tool with other collaborative authoring efforts and underline why both the storytelling medium "emergent narrative" and our particular approach to authoring are better suited for massively collaborative authoring. © 2008 Springer-Verlag Berlin Heidelberg.