There are some basic guidelines when approaching theatrical photography.
Firstly – and I can’t emphasis this enough – do not use flash. Not only is it distracting to the performers, but it completely negates all the hard work that goes in to lighting a production. One of the key elements of any production is the lighting design, adding drama and effect, concentrating the audiences view and creating mood. Using a flashgun completely obliterates this – quite literally in a flash. Stage lighting is typically adequate enough for any respectable SLR. I often choose to shoot in manual mode and open up the lens aperture, whilst selecting an appropriate ISO speed for the lighting conditions. Otherwise you might want to use Shutter Priority and a speed setting (ISO) that gives you a shutter speed roughly equivalent to the focal length of the lens. On my Nikon 600 you can also set the ISO to auto in shutter or aperture modes.
Secondly, stay out of the way. A lot of work goes in to theatrical productions and the performers and backstage crew need to focus on their roles. Any distraction from a photographer should be kept to a minumum. Wear dark clothing so that you can blend as much in to the background.
If possible, photograph a dress rehearsal – so you won’t distract any audience and you will be able to cover more shooting angles.
Finally, shoot both wide and close up. Show the characters in relation to the set and each other.