Part of the work is determining through what instrument you are playing. Actors are physical, olympian storytellers and we should be able to create entire landscapes with nothing.
When I was a younger actor, I would try to keep it serious all day. But I have found, later on, that the lighter I am about things when I'm going to do a big scene that's dramatic and takes a lot out of you, the better off I am when I come to it.
Making a film is so hard that if you don't have your main actors going along with the ride with the rest of the crew it can make your life very difficult.