This case study analysed the circumstances during a one-year work placement in which a pre-service teacher showed professional development, with a focus on the impact of her interactions with various interlocutors. The study was conducted within the framework of the cultural historical activity theory (CHAT) and of activity theory with its methods for the clinical study of activity. The results showed intra-psychic conflicts. produced by interactions with trainers, peers, experienced colleagues, and students. These conflicts prompted the development of the pre-service teacher's professional activity through the construction of new goals and new motives for action (development through sense), and the construction of new operations to reach these goals (development through efficiency). This study thus opens the way for creating training programmes that favour the development of teachers' power to act. It provides a source for transforming traditional mentoring methods in teacher training. (Contains 2 tables and 1 note.)