Jack O'Connell has revealed playing Olympic athlete Louis Zamperini was "the biggest challenge" of his career to date.
The former Skins star portrays the sportsman and World War II hero in the drama, directed by Angelina Jolie and written by the Coen brothers.
"Probably in a singular context the biggest challenge of my career," the 23-year-old told Collider.
"It was the opportunity to portray such a monumental human being who was true to life and is still alive now. It was that opportunity that made the job irresistible to me."
The film, which also stars Garrett Hedlund, Jai Courtney and Domhnall Gleeson, tells the true story of Louis, the former track star who spent 47 days afloat in the Pacific Ocean and then as a prisoner of war, after washing up behind Japanese enemy lines.
"To be at the age I'm at and to have a mature enough state of mind to be able to appreciate to the best I can quite selflessly, to be able to appreciate what he must have had to... at a similar age to myself, what he went through," he said.
Jack, who played cheeky chap Cook in E4 drama Skins and has been promoting 71 at the Berlin Film Festival, heaped praise on his director.
"Angelina Jolie has to be one of the most empathetic actor-directors I've ever worked with, and we had a sterling cast and the studio took a risk casting an unknown as their Louis so I really relished that opportunity," he said.
"Every adversity I was trying to face, she's done time and time again and I certainly grew and strengthened as an actor or a human. "
Unbroken is scheduled for release in December.