When "The Iron Claw" was filmed, Zac Efron was in his mid-30s and no stranger to beefing up for roles. However, the professional wrestling training was unlike anything he had experienced before. USA Today reports the term Efron used was "wildly uncomfortable," when talking about learning the actual ropes of wrestling. The gym regimen also took a toll on Efron. "Just motivating yourself to get up and train and push heavy weights can get pretty tedious," he said.

Meanwhile, his co-stars Harris Dickinson and Jeremy Allen White struggled with different aspects. White confessed that "eating something every two hours" was far from his usual routine, which had previously consisted of training in a professional restaurant for his role in "The Bear." Dickinson felt that the process was abnormal for him, too. "The physical transformation was hard. I'm not naturally athletic like that," he said. Dickinson explained to Variety that to transform their bodies, they not only had to lift weights and train as a wrestler but also hit a daily caloric intake that hovered around 3,000 calories.

In the end, White reportedly gained 40 pounds in total, according to The Sun. Reflecting on his experience, he told Esquire, "It's really just gross," but admitted that Efron seemed to be cut out for the role. "In terms of a physical transformation, I think he kind of blew us all out of the water. He looks really amazing in this thing," White said.