I've shot several plays in the same auditorium so I know what settings to start out with. The only variables being what lighting is used in the various scenes. An aperture of f/2.8 and ISO 3200 seems to work well for all but the darkest scenes. Here are a couple of shots followed by an embedded slide show.
Something different this time was that instead of using off camera flash and a reflector for head shots we simply used a single spotlight against the closed stage curtain. Much quicker and easier and I like the results.