Let's say you have two elements which shall get different lighting. Of couse each of them is in its own group layer:
- A group/bone layer
- - some stuff
- - some other stuff
- B group/bone layer
- - the stage stuff
Add one layer with a dark (black or coloured) rectangle covering the whole frame. Setup:
- A group/bone layer (hide all)
- - dark rectangle layer (mask this layer)
- - some stuff (add to mask)
- - some other stuff (add to mask))
- B group/bone layer
- - the stage stuff
Set the dark rectangle layer to "multiply" and its opacity to 50% for a start. Render. You may now animate the opacity from 0% to 100% to get a "lights fade out" effect on the crowd.
Tip: I put stuff in groups a lot for these shots, just because I only have to adjust one group layer's mask settings for this, instead of dealing with dozens of character layers.
I did something like that last year:
http://www.slowtiger.de/examples/saal.html. Here I cut a hole into the rectangle and moved that layer to get the effect of moving spotlights.