Actor Jared Leto, who played at South Africa's My Coke Fest shows with his band 30 Seconds To Mars, will never bulk up for a film role again after he put on 27kg "gorging and force-feeding" himself for a part that left him unable to walk.

Leto, best known for roles in films like 'Alexander', 'Fight Club' and 'Lord Of War', is still suffering the side effects of piling on the weight he gained to play John Lennon's killer.

Over a year after the film 'Chapter 27' debuted at the 2007 Sundance Film Festival, Leto is afraid he won't be able to regain his original physique.

"I'd never do it again; it definitely gave me some problems," the 36-year-old said of bulking up to play Mark David Chapman.

"I don't know if it was gout — but I had a definite problem with my feet. Towards the end of the shoot, one of the glaring issues was the pain I had with my feet. I couldn't walk for long distances; I had a wheelchair because it was so painful.

"My body was in shock from the amount of weight I gained. It took about a year to get back to a place that felt semi-normal. I don't know if I'll ever be back to the place I was physically."

But Leto believes the weight gain was vital for capturing Chapman's spirit: "It was important to make that transformation because I thought his physical representation of himself was an indication of who he was.

"And it changed everything about who I was — the way I walked , the way I talked, the way I felt about myself, the way people treated me.

It was an interesting experience. It was interesting to see people reactions — particularly people I knew but hadn't seen in a while and who obviously thought I'd let myself go."

In 1980 Chapman shot Lennon dead outside the former Beatle's apartment in New York.