Sitcoms with a live audience typically use a three-camera format. This means that instead of shooting several versions of each scene (which would ruin the flow for the studio audience and take many hours), they shoot the entire show with three cameras. Typically in three-camera, one camera is focused on a closeup of each of the two primary actors in the scene, and the third is the long shot which includes multiple actors and the set. So an actor doing a close-up just has to look into the camera that is focused on him or her and the shot will be picked up. After the shoot, the video editor puts together the camera shots to creat the finished show. Next time you watch a sitcom, try to figure out which camera is being used for each shot. You should be able to pick them out fairly easily.
Typically the actors do not know for certain which camera shots will be used in the final version of the show, since the video editor will put the shots together after the show is filmed. However, sitcoms are carefully blocked, which means that before filming in front of an audience, the movements of both the actors and the cameras are mapped out. So during a rehearsal, the director may have asked a particular actor for a reaction shot, and made sure that a camera would be there to capture it.
It’s also possible to do “pick-up shots”, which are shots that are filmed after the primary filming to get a particular look or reaction.
Cameras with lights are used for live broadcasts, such as T.V. newscasts. Of course these programs will not be edited, so the light is there to tell the performers which camera’s shot is going out to the audience at that moment.