Sometimes people don't rehearse at all, but you might have had a chance to rehearse for a few days, or even more than that. Then in the moment when the camera's running is the only time it matters. So whatever you've discussed or thought about or discussed with the director, the other actors, hopefully there's a part of the experience that you've left open so that only in that moment the camera catches it. That's of course the hardest thing to do because everything is planned and you have thought about it in advance.