In the new Men's Health, "Thor" star Chris Hemsworth says he gained 20lbs of muscle to play the hammer-wielding superhero in the flick -- but he's definitely not the first star to drastically change his weight for a role.

From Christian Bale's shockingly thin appearance in "The Machinist," to Renee Zellweger's curvier figure in the "Bridget Jones" movies, check out who else has packed on or dropped the .lbs for a movie!

"Thor" opens May 6.

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