Medical educators constantly make decisions on when and how to intervene. Current literature provides general suggestions about types of teacher interventions. Our study aims to specify that knowledge by describing in detail the actions teachers do when intervening, the interactional consequences of those actions, and how these relate to teacher roles in group discussions. We collected all first teacher interventions (n = 142) in 41 video-recorded group discussions on experiences from practice at the Dutch postgraduate training for General Practice. We analyzed the interventions using Conversation Analysis. First, we described the timing, manner, actions, and interactional consequences of each intervention. Next, we inductively categorized actions into "types of actions." Finally, we analyzed the distribution of these types of actions over the group discussion phases (telling, exploration, discussion, conclusion). First teacher interventions were done at observably critical moments. Actions done by these interventions could be categorized as moderating, expert, and evaluating actions. Moderating actions, commonly done during the telling and exploration phase, are least directive. Expert and evaluator actions, more common in the discussion phase, are normative and thus more directive. The placement and form of the actions done by teachers, as well as their accounts for doing those, may hint at a teacher orientation to intervene as late as possible. Since the interventions are occasioned by prior interaction and responded to in different ways by residents, they are a collaborative interactional accomplishment. Our detailed description of how, when and with what effect teachers intervene provides authentic material for teacher training.