Interactive scenarios are often represented as branching-like systems. This representation complexity is increased as options left to players grow. Each fork in the story force designers to define all possible choices and their following situations. We propose a formal model of scenario to ease designers task while granting the same freedom to players. In this model each entity is modeled as an automaton. These automata are then combined to build the scenario as all entity's actions sequences. The right sequence is then unfolded on line, while the player experience the story. The structure of the narration is guaranteed by properties verification on the model. These properties are set by designers to provide an interesting narrative frame to players. We implemented this method on an interactive game prototype linked to an expert system in charge of unfolding the story.