When I'm on assignment it's important to capture the ambiance of a place; I never want to overpower the scene or the "feel" of the location with my lighting. I start by testing the available light, then decide on how best to supplement what's there with additional lighting to highlight what we most want to show in the final image.
Some situations are harder than others to light. This Situation Room at an internet security company in Boston is a case in point. It was important that we see the people but the room itself was kept dark so the workers could more easily read their screens; we also needed to make sure we could see the screens of each computer. Multiple lights and lots of tricks were used to keep this scene looking natural; leaving the desk lamps on added that soft, warm glow of the ambient light. ISO 400, 1/50 sec at f6.3, 18mm lens.