Actor Miles Teller impressed himself with his physical transformation after slimming down and building hard muscle to play a championship boxer in new movie Bleed For This.

The Whiplash star admits he wasn't in the best of shape when film producers first tapped him for the role of real-life fighter Vinny Pazienza, so he threw himself into a strict training plan and adopted a hardcore diet to get ripped.

The results helped him pull off his portrayal of Pazienza, who mounted a stunning comeback after suffering severe spinal injuries in a near-fatal car accident in the early 1990s, and Miles insists he marvelled at his peak fitness during the shoot.

"I filmed this movie not that long after (2014's Divergent)," he told breakfast show Today. "I certainly don't think I'm the guy studios would call in to be a five-time world champion American boxer, I just wasn't. We filmed this two years ago, but I lost 20 pounds and got down to six per cent body fat, and then I had to gain 15 pounds back while we were filming."

Miles even refrained from meeting the real Pazienza until cameras were about to start rolling.

He explained to Business Insider, "I kind of wanted to keep my distance because I just felt like he would have been embarrassed by me before I got in shape. I really needed eight months, honestly, to get into the shape and do all of those things and to learn the boxing. So I was really intimidated by Vinny's legacy and him himself. But once I got to Providence (Rhode Island) he was the first person I went to go see and that was such a surreal moment to finally meet the guy that you had been thinking about portraying and been obsessing over.

"He would come on set a little bit, and that was great. I knew he would be proud of what we were doing."

However, the 29-year-old confessed he didn't keep up the healthy eating and exercise regimen for long: "I wish I still looked like that!," he laughed. "That was a fleeting moment in my life."