The Australian actor plays the Nordic superhero in the upcoming self-titled blockbuster, directed by renowned thespian Kenneth Branagh.
Movie bosses ordered the star to hit the gym and gain as much muscle as possible before filming started, but Hemsworth went too far with his workouts and became so heavy he had to have his costume re-sized.
He tells the Los Angeles Times, "I got the part and immediately started looking at the comic books, and the guy is 500 pounds or something and looks like Schwarzenegger. And I thought, 'OK, I'm not gonna get to that.' But I have to get bigger. (The filmmakers) kept saying 'Yeah, get as big as you can, as big as you can.'
"And then I came back right before Christmas, just before we started the movie, and had a final sort of camera test and put the costume on and within a couple of minutes, my hands started going numb.
"And everyone was like, 'Yeah, that's not cool.' And I started getting pins and needles and you'd touch the skin and the blood wouldn't return to that area so quick. And I thought, 'OK, it doesn't fit'."
Hemsworth's costume was altered to fit his new shape, and the actor was ordered to "pull back" his gym sessions to prevent any more problems.