In this text, we argue that user modeling and system modeling should be taken into account at the same time to allow cooperativity: they are two sides of a same interaction. We focus on interlocutor modeling (the human-human homonym of user modeling) and develop three situations where interlocutor modeling is crucial: when there is a need of transforming the beliefs of the cooperator (when there is a knowledge gap or a resistance), when the interpretation of the cooperator's communication act is ambiguous and relies on implicit meaning, and when there is a specific need need of anticipating the other's understanding (in risky or irreversible situations).