This is a tough one. As a general rule, I like to have my students perform the exercise with as little interference as possible. I do enjoy role playing as the victim though and this provides me a unique perspective on the drill. You may need to step in to clarify a technique on occasion. We will often demonstrate various skills before having the students perform. Always ask yourself if your involvement is necessary to further learning or not.
I do provide several really challenging scenarios/exercises. These really challenge my helpers and students and make them think. Often, my helpers are included as extra participants in scenarios in the more difficult roles. This can be fun and very instructive for them and the students. This is also a great recruitment tool for helpers and eventual instructor candidates.