Interactive storytelling can be based either on explicit plot representations or on the autonomous behaviour of artificial characters. In such a character-based approach, the dynamic interaction between characters generates the actual plot from a generic storyline. Characters' behaviours are implemented through real-time search-based planning techniques. However, the top-down planning systems that control artificial actors need to be complemented with appropriate mechanisms dealing with emerging ("bottom-up") situations of narrative relevance. After discussing the determinants that account for the emergence of narrative situations, we introduce additional mechanisms for coping with these situations. These comprise situated reasoning and action repair: we also illustrate the concepts through detailed examples.