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.