Zac Efron underwent a "shocking" transformation for 'The Iron Claw'.

The 36-year-old actor plays real life wrestling legend Kevin Von Erich in a new biopic about his family's rollercoaster dynasty, and he threw himself into the role from getting in ridiculous shape to rocking a bowl cut in revealing attire.

He told Entertainment Tonight: "It was shocking at every point of the transformation.

"I remember Harris [Dickinson] and I were walking out of hair and make-up and it was like the first time we had to wear the full kit, and we were both thinking, 'What the hell are we doing here, man? How did we get into this?'

"But I think it ended up looking really good and we had a great crew helping us out with that. The hair and make-up team was epic."

Efron took inspiration from Von Erich himself as it came to making sure he did him and his family proud.

He added: "This movie's a true story, and that man, Kevin Von Erich right there, he really motivated me to get in shape for this movie.

"The physicality he brought to the ring every single day was really unique. It changed wrestling.

"I knew that was going to be the hardest [thing] for me to master and get right. So, I put everything I had into it."

Thankfully for Efron, he's had the stamp of approval from the grappling pioneer.

He revealed: "He told me he was happy with it last night so I'm stoked! I cried a little bit, if I'm going to be completely honest."

The movie follows the Von Erichs, led by father and coach Fritz Von Erich (Holt McCallany) and his sons Kevin, David (Dickinson), Kerry (Jeremy Allen White) and Mike (Standley Simons).

The wrestling dynasty dealt with highs and lows in the most extreme ways, Kevin's brothers all dying before their father, sparking the concept of the 'Von Erich curse'.