And at the most intrusive level you can stop the role play, bring the players out of role, discuss and return to role. The goal ought to be to use the least intrusive intervention that will accomplish the job. Another point worth making is that it is not only the instructor who can take the role of intervenor. Ask Leading Questions and Let Them Talk. Telling people things is not an effective way of getting them to learn them.
People learn best from what they themselves do, think and say.
Therefore, in debriefing an exercise, let the players explain what they learned. If it's necessary to tell them something, tell them in a way that they are likely to hear you. Criticism gets ignored. Phony flattery is eventually discounted. You can lead and teach without coercion.