This article reports on a study into how collaborative contexts influence the professional development of an early-career primary teacher, Julia. We describe the process of change by which Julia manages to make her planning to teach mathematics more flexible so as to adapt to student difficulties, and we analyse the role that joint reflection plays in promoting this change. In order to understand the "how" of this influence, we carried out an analysis of the interactions within the group from Julia's point of view, following a dialogical approach to discourse. We believe that it is in and through the interactions that Julia constructs her interpretation of the opinions, critiques and suggestions expressed. This interpretation conditions the extent of her involvement and moulds the influence of the context on her professional development. The presence of skilled collaborators proved decisive in promoting this development.