It's common for actors to obsess over a role, wearing the persona like a disguise while filming so they really sell the character on the screen.
But actors who really commit to a role are willing to put their physical health on the line to embody a character, gaining or losing an unhealthy amount of weight to help them get into the role.
Christian Bale has both lost and gained to properly fit into his roles-he lost over 50 lbs. for his role in The Machinist and gained 43 lbs to play Irving in American Hustle, a gain that resulted in two herniated discs.
Kevin James is known for being a “big guy”, and he weighed in at 285 lbs. before training for his role in Here Comes The Boom, where he played a high school teacher turned MMA fighter.
After training intensely for the role he'd slimmed down to 218 lbs. and was clearly in peak physical condition in the film.
See 19 Actors Who Went Through Insane Physical Transformations For A Role and 10 More Celes Who Underwent Extreme Physical Transformations For Film Roles