Watch the New Trailer for Angelina Jolie's World War II Epic 'Unbroken'

Jack O’Connell’s physical and mental resolve was tested often during the making of the upcoming World War II epic Unbroken. “Contrary to the title, it broke me on several occasions,” he tells Yahoo Movies with a laugh. “But that’s the point, isn’t it? It had to break me. Otherwise I was just out there pretending.”

In the new trailer above for the period drama directed by Angelina Jolie, we see just how far O’Connell was pushed. (We’ve also got the two exclusive poster reveals below for the film that will hit theaters on Dec. 25.) The actor — who’s currently starring in the acclaimed prison movie Starred Up — plays the real-life Olympian and war hero Louis Zamperini whose story was recounted in the Laura Hillenbrand book the movie is based on. A track star, Zamperini competed in the 1936 Olympics where he was good enough to draw the attention of Adolf Hitler. He later enlisted in the Army Air Force during World War II and survived a plane crash in the Pacific, 47 days adrift in a life raft, and two brutal years in a Japanese prison of war camp. The real Zamperini passed away this July at the age of 97.

“I was in awe. Three times I met him,” O’Connell says of the man he portrays. “I hear before he passed away, Angie showed him some footage, of which he approved of, which means the world to me and to us all.” The actor also adds that Zamperini gave him an invaluable image to carry with him during filming. “He always wanted to cross the finish line in whatever he was doing,” O’Connell says. “Whether it was on the raft or in Japan as a prisoner of war. He was always trying to get to the end of something.”

It was this spirit of perseverance that O’Connell tried to emulate during the most grueling moments of the shoot, especially as the production recreated Zamperini’s time lost at sea. O’Connell shed an almost dangerous amount of weight for the scenes, though he seemed more bothered by the contact lenses he had to wear every day to make his eyes look bloodshot. “I don’t let anyone poke me in the eye willingly,” he says. “That was a hurdle I had to get over.”

Of course, for all that he went through during the production, what separated O’Connell from the man he was portraying was the ultimate safety of a movie set, where shark attacks are simulated and trailers await. “I wasn’t alone, I wasn’t ever suffering more than Louis,” O’Connell says. “I was fairly certain that I was going to survive — at least I was going to be alive at the end of the shoot. And that was something he could never guarantee for himself.”

Watch a video about the #IAmUnbroken campaign below: