I don’t think this is by any means unique to education…except insofar as the inexperienced cast may display more of a lack of discipline than is strictly speaking desirable. But that happens with chummy casts in theater communities the industry-over. Recently I worked on a new comedy musical. It was super fun. The cast gelled right away, I became good friends with many of them, the production team was great to work with, the subject matter was stellar with an all-bets-are-off sort of design (all the color! All the unicorns! All the confetti!). It was a recipe for delightful chaos.
I generally found it was best to pick my battles. If the MD is giving an actor music notes, it’s quiet on set. If the director is deliberating with his assistant and people take 30 seconds to breath and joke, cool. If a scene disintegrates into fits of uncontrollable laughter because of an acting choice, I’ll let it play out for a minute until we can get things under control…because it can be hard to come down. Once I even had to call a five to let people regroup, but that was drastic.
Maybe the most important thing is to respect the spirit of the show. In a project like the one described above, the silliness (within reason) could add to the creative output. In a more serious drama, it could hinder it. Also keeping in mind mitigating circumstances—for example, when I worked on Streetcar and we were blocking the rape scene there was a certain amount of levity on set and while we treated the discussion with due seriousness, I felt I had to consider the difficulty of the subject matter and respect the actors’ coping mechanisms.
So in answer to your actual question, I would engage the cast as long as it furthers the goals of the rehearsal. Also make your expectations (whatever they may be) clear from the get-go. If it's too difficult to maintain authority and be their friend, let them know you'll be their friend outside the rehearsal room. *my ten cents...shutting up*
