In this paper, we propose, and evaluate, a proactive reflective activity for mathematics student teachers in which they consider mathematical content and its teaching in highly specific classroom situations. Student teachers identified, scripted and reflected upon critical classroom incidents soon after their first block of school placement. Twelve such scripts were produced and then discussed by seventeen student teachers in group and plenary sessions. Discussions were audio-recorded. Scripts and discussions were analysed according to a typology of four characteristics that emerged in our prior research: consistency between stated pedagogical priorities and intended practices; specificity of the reflection to the classroom situation reported in the scripts; reification of pedagogical discourse that the student teachers became familiar with during the programme; and, reification of mathematical discourse that the student teachers were familiar with through prior mathematical studies. Here, we exemplify student teachers’ insights that emerged from the analysis of the scripts through the typology of the four characteristics and we observe that their insights mirror the complexity and richness of the mathematics classrooms they face. Our examples, and their evaluation through the aforementioned typology of the four characteristics, illustrate the potency of student teacher participation in producing, and reflecting upon, individually and collectively, on critical incidents of their inaugural experiences in the classroom.