The hardest thing for a film actor, especially if you are in a lot of the film, is sustaining energy for the entire length of a production. It's quite tough. With acting, it's not the same as directing. Directors work the exact same hours; directing is incredibly exhausting. The only difference is that directors aren't required to have bursts of energy and focus. They're probably focused the entire day. Actors have this thing of "stop/start." That can be quite draining, actually.