Zac Efron found it “very uncomfortable” standing around in his underwear between takes on the set of ‘The Iron Claw’.

The actor, 36, plays wrestler Kevin Von Erich in the movie alongside Harris Dickinson, ‘The Bear’ star Jeremy Allen White and Stanley Simons as the rest of the wrestling Von Erich siblings, David, Kerry and Mike.

Zac told Metro about the biggest downside of the part: “One of the jarring moments that I wasn’t expecting was when the wrestling stops, or there’s a cut, or in between set-ups or takes – you’ve just got to stand there with a whole crowd around, in your underwear.

“And there’s nothing really going on.

“That’s a different kind of feeling. I’ve never experienced that one before.

“You just kind of gotta, like, where do you put your hands? On your hips? You can’t help it – there are people everywhere, you can’t look up and have somebody not looking back at you! So that was a bit unnatural.”

Before also admitting the process was “very uncomfortable”, Zac added: “You get used to that, too!”

The actor gained 15lbs of muscle while working out multiple times a day for the film, which he said was another downside to the project.

He added: “You’re sore a lot. It’s called delayed onset muscle soreness, and they get pretty debilitating.

“It’s hard to move around and you just feel old.”

The movie shows how the Von Erich brothers’ dad Fritz – who had been a world champion wrestler in his own right and created the ‘iron claw’ hold – was a brutal dad who drove his boys to their limits.

Zac said about how wrestling in the ring allowed him to push through his pain of training for the film: “I think when you're in the ring wrestling, you kind of become invincible.

“I don’t know what I could relate it to other than skateboarding when I was a kid – but you can do incredible things and the pain is dissociated from your mind, until you get home later that night, and you’re like, ‘What did I do?’ and your ribs are broken or something.”