“Once you get close you’re just always going to see things other people don’t,” the actor, 29, tells PEOPLE. “You get in this rude state with your body. Even though you know you’re in great shape you’re thinking, ‘This could be more, this could be more muscle.’ You think about other actors who have played boxers and you want to look how they look.”
Teller wanted to look as similar to Pazienza as possible after he signed on to tackle the true story of the world champion boxer. Pazienza mounted an astonishing comeback after nearly becoming paralyzed in a car accident.
In 2014, when he got the role, Teller had mostly played high schoolers and college students, and wasn’t in fighting shape.
“I was just not the actor people would think about to play a boxer,” he says. “I was not in the physical shape, and so the ‘Oh s—‘ factor started immediately. I got a scale and I was like 20 lbs. overweight from where I needed to be, and body fat-wise I was at 19 percent and I knew I needed to get down to about 6 percent.”
Teller got right on a strict diet for eight months, and “cut out everything.”
“I was kind of eating like a rabbit,” he says. “No bread, no alcohol.”
For more on Teller’s diet, pick up this week’s issue of PEOPLE, on newsstands Friday.
The actor says he wasn’t expecting how difficult the diet turned out to be.
“Maybe I was naïve in thinking — oh yeah, you do some sit-ups and you get abs. Well you can have abs and you can look good, and I call that stuff glamour muscles — you can look good aesthetically, but can you box for 12 hours while cameras are going and still be able to act and think and not pass out?”
Teller hit his lowest weight for a scene where Pazienza shows up for a weigh-in donning just a leopard-print thong.
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“I knew, in theory, for that scene, Vinnie was at his lightest weight so I should look the most toned. And I didn’t drink any water or liquids for two days before that. I remember the night before I was in the steam room trying to get out everything,” he says.
Luckily Teller was able to add back 15 lbs. towards the end of filming, when Pazienza moves up a weight class, but he plans to keep the principles of the diet going.
“I do maintain the diet regimens,” Teller says. “I know what’s good for me and what’s bad for me. I just got such great knowledge it would be foolish for me to just throw that all away.”