In previous work, we designed, implemented and analyzed a teacher-training scenario based on role-play, which immerses preservice teachers into a situation involving a primary school teacher helping a pupil with some mathematical task (Lajoie et al., 2019). We highlighted some potential offered to the teacher trainer to bring out, in preservice teachers, mathematical, didactic and pedagogical knowledge but we came to the conclusion that making the most of this potential involved demanding work on the part of the teacher trainer. In this paper, we investigate this dimension within the activity theory field, more precisely the way a teacher trainer who did not participate in the design of the scenario implements it, in particular the way she manages the collective discussion phases.