Believe it or not, you've got to light a dark room. The hardest part about filming in a dark room is convincing the audience it's dark, because they know that if you photographed it, then there's light there. The thing that mucks up the illusion is the technology. If there's not enough light then you have to print up the negative (resulting in "dancing grain"-a giveaway that the DP may have been living on the edge and fallen into an exposure abyss) or turn up the gain on the electronic camera (resulting in more noise, blacks that milk up and blown out whites).

Start by lighting the room flatly, but to a decent stop. Don't worry about how bright it looks to your eye. You're telling the camera what it sees, not the other way around. You're lighting the room up, so that you have somewhere to go with the stop on the lens. Now, include some very sourcey hot spots (4-6 stops above ambient), like you would get from light leaking around doors, through cracks in the curtains, whatever. Make sure you also have a few very dark spots(2-3 stops below ambient) in the background somewhere. The bright spots help explain why there is some light in the room and both the dark and light spots help provide seperation from the background as your actor passes in front of them.

Now stop down 2 stops below what you think would be appropriate exposure. Your ambient is now 2 stops underexposed-a dark grey, your dark spots are now 4-5 stops underexposed-deep grey or black, and your hot spots are nice and hot, but not so hot they blow out- 2-4 stops overexposed. You won't get noise, because the room is lit, but it'll look dark because you changed middle grey (and if you don't know what middle grey is, go out and get Ansel Adams' books explaining the Zone System.) If you need a little more light on your actor, hang a china ball on the end of a stick and attach it to a dimmer. Keep it really dim, you don't want shadows moving in your background as you move the ball around, and you really want just enough light on the actor to register their eyes. You may even want to wrap most of the china ball with black to keep the extra light from flying around the room and mucking up your darkness.

ps. That 5-10 minutes before the sun goes down completely is called Magic Hour (it's actually got a name) and it's not really an hour, but the 20 minutes or so after the sun goes down but before the light goes away. For a movie shot almost entirely in Magic Hour watch "Days of Heaven" (they spent the whole day setting up many shots, then leapfrogged from one to the other as fast as they could in 20 minutes).