In this paper I shall analyze interaction sequences in which the interactants explicitly thematize and problematize whether the preparatory conditions of a speech act in a specific enunciation situation can be considered as satisfied or not. In these sequences the interaction develops an open, and consequently, observable contextualization process. In order to describe the dynamics of this process, central concepts and methods of Mental Space Theory will be applied. Contextualization will be characterized as a continuous process of construction, connection and modification of mental spaces.