It depends on how tired we are, and whether anyone else is home at our respective houses. If we're alone, usually "watch a movie" means "have sex". Because we are both horny as fuck all the time, and can't stand being cuddled up together for two hours straight without at least a little fooling around.

EDIT: Oh, it also depends on the movie - I've seen four basic ways it plays out. If it's some chick flick she is making me watch, I'll usually initiate the action. If it's some brainless action movie, she will come onto me. If it's a dumb movie neither of us are interested in after the first few minutes, we'll both disregard the movie. Occasionally it will actually be a good movie both of us really want to see, and then we will actually watch it all the way through without interruption.