Diagnosing teachers are teachers who perceive diagnostic information about students’ learning process, interpret these aspects, decide how to respond, and act based on this diagnostic decision. During supervision meetings about the undergraduate thesis supervisors make in-the-moment decisions while interacting with their students. We regarded research supervision as a teaching process for the supervisor and a learning process for the student. We tried to grasp supervisors’ in-the-moment decisions and students’ perceptions of supervisors’ actions. Supervisor decisions and student perceptions were measured with video-stimulated recall interviews and coded using a content analysis approach. The results showed that the in-the-moment decisions our supervisors made had a strong focus on student learning. Supervisors often asked questions to empower students or to increase student understanding. These supervising strategies seemed to be adapted to students’ needs, as the latter had positive perceptions when their control increased or when they received stimuli to think for themselves.