I'm also planning something with non-actors, though not perhaps as many as on Einar's project. Jeff, I find your ideas helpful. I was wondering though because the situation is rather chaotic, is there any advantage in actually turning the camera and trying to film that rehearsal. In your experience can the rehearsal ever come across better than the performances that follow?
I can't speak for the efficacy of shooting rehearsals. I've only shot film and it can't be used in the same cavalier manner as digital can in that sense, especially on a budget. If I may be so bold, I'll use that as a selling point for shooting on film though. Whenever I've told actors or non-actors alike that we were shooting actual film, they take it way more seriously when action is called than they do if it's just digital.
I do find though that it's better to rehearse at least once and tweak what you see and immediately shoot after making those changes rather than wait for any length of time between the rehearsal and the shot. Actors tend to be patient and understand the time it takes to prepare a shot. Non-actors will lose interest the more they stand around and wait, especially if they're volunteers and aren't being paid. I brought in 20 volunteers for a scene and had to be on my toes not to waste their time at all, and at the same time make it fun for them even though I really wanted to pull my hair out because of the details of the scene. After about two hours, you'll start to get whispers in your ear from one or two that they "have to leave" for whatever reason, which is their kind way of saying that they're bored and want to go home. I was able to get through the scene with losing only 4 people by the end of a four hour night shoot. Not bad for volunteers giving up their evening, most of whom had never met me before.
One more trick that works well, although it's a little devious: Non-actors don't understand lens focal lengths. You can be on an 85mm doing a close-up of the actual actor in the scene who happens to be amongst a crowd, but because your camera is pointed in the general direction of the crowd, they think they're on camera during the shot. Never let the crowd know they're off camera, even if you are doing a close-up and only see one person's face.