I have worked as a gaffer on the TV reality show Project Runway for the past five summers. The practice to lighting in live locations is to be subtle and to use the light to fill in the faces, so that the subjects don't have "raccoon eyes". I lit people at the UN with five cameras shooting - one doing steadi-cam wide shots. So the lights had to be backed up pretty far.
For your theater shoot you have several possibilities. It all depends on what you are filming. Are the two people looking at each other or out toward the audience? How wide are your shots? These are major question that will effect the lighting set-up.
One way to light it is to simply embrace the theatrical look of the theater. You can use the previously mentioned source-4 profile spots, but use at least 36 degree if not 26 degree lenses. Place them like theater lighting - 45 degrees off to each side and along the walls. Raise them up a few feet above eye level. The farther away you are the tighter the degree lens you would want to keep intensity, get the throw and not spread too wide that it just looks like a messy wash. You can use a light frost to soften the light and run the barrels out to defocus the hard edges of the beam. I would then recommend adding side lights, using fresnels or PAR lights. Scrim these down, but allow them to kick off the subjects and separate them from the background. You could even use light color gels on these to convey the theatrical location. Don't use green or yellow - stay with the more calmer colors such as lavender, sky blue or bastard amber. You could easily use 1/2 CTB and 1/4 CTO.
Another way is to use softlights, such as 4 bank by 4ft Kinoflos set off to each side of the stage - becoming cross key lights. Each light backlights the person closest to the light and key lights the person farther away. Now, further back in the house, use fresnels with diffusion, such as opal or 251, on the inside of the barndoors. You could also use PAR 64 lights with diffusion, but the fresnels will project farther. Last, don't be afraid to add a kinoflo on the floor, covered with diffusion such as 250. You could use two, one for each subject. Put the kino on low output (2 lamp output for a 4 bank unit). Keep it very diffused so that it fills in under the eyes and chin without throwing monster movie shadows up on the faces.
In out my book "Lighting for Cinematography" new from Bloomsbury Press, I have an entire chapter on non-fiction lighting which may help you. The book is available from Amazon and my website www.lightingforcinematography.com
Good luck and I hope this helps or inspires you a bit.
Edited by David Landau, 09 July 2014 - 08:39 PM.