Some actors and actresses take their job very seriously. They really go the extra mile, even deciding that their physical health is worth risking, as long as it is for the good of the movie.
1. Christian Bale
This is far from being a surprise. Anyone that plays Batman is inevitably going to start believing that they are the real Batman. Of course, Bale did his own stunts long before the Batman days. Now it is just that much cooler.