But Mark Ruffalo has been through the pain of toning up his body before - he followed in his dad's footsteps and won medals as a student wrestler.
He tells E!Online, "They (film bosses) want me mean and lean, but they don't want me big and buff. I gotta keep building and building and building. You have to eat to build muscle."
"I was a wrestler. I'm used to starving myself. It's easy for me. The last two weeks you just walk around spitting into a cup."