Jeremy Allen White on Gaining 40 Lbs. of Muscle and 'Confidence' For 'The Iron Claw' Role (Exclusive)

The actor is playing Kerry Von Erich opposite Zac Efron for 'The Iron Claw.'

Jeremy Allen White has confirmed that he gained a whopping 40 pounds of muscle for his upcoming movie, The Iron Claw, and says he also gained a certain "confidence" in the process of filming.

Talking with ET's Cassie Dilaura Wednesday at the premiere of The Iron Claw, White, 32, explained the phenomenal transformation he underwent to become a member of the famed Von Erich wrestling family. 

"I was eating a lot, like, a pretty tremendous amount," the actor shared. "Just eating as much as I could and just trying to lift weights, no cardio." 

White said going without cardio workouts like running and jump roping was difficult for him as he likes to run and struggled to sub out those workouts with lifting "heavy things." 

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The other thing that he had to get over was the hair styling and costuming for the movie, which took the actors immediately back to 1970s and 1980s Texas with big bowl-cut wigs, cut-off jean shorts and sleeveless tee-shirts. It was a look that White said took a minute to adjust to. 

"I wasn't confident immediately but then I think we all look so different with the wardrobe and the hair and I think that finding that comfort in something that looked so far into us, I don't know, it was a real gift," White said of finding his confidence while shooting. 

The Shameless alum is playing Kerry Von Erich while the wrestler's brothers -- Kevin, Mike and David Von Erich -- are being portrayed by Zac Efron, Stanley Simons and Harris Dickinson, respectively. 

Since the movie centers around the American wrestling family, many of the scenes involve shots of White and his co-stars in their wrestling shorts and nothing else -- another facet of the film that he had to learn to adjust to.

"I was not confident," White said, laughing. "I got confident, I guess. Seven days of running around in your underwear, you get used to it." 

White also joked that while Dickinson took his own wrestling shorts home at the end of filming, he grabbed Efron's shorts as a memento of their time on set. 

The Iron Claw premieres in theaters on Dec. 22.