At the start of The Way Back, a title card appears on screen offering a dedication to a group of men who escaped a Serbian gulag during WWII and traveled over 4,000 miles, across the Gobi desert and Himalayas all the way to India, in order to reach safety. And they did it on foot.

Adapted from Slavomir Rawicz’s novel, The Long Walk: The True Story of a Trek to Freedom, the film stars Jim Sturgess, Ed Harris, Saoirse Ronan and Colin Farrell, and is directed by Peter Weir, leader of Australian New Wave cinema, whose previous films range from Picnic at Hanging Rock to Dead Poets Society to The Truman Show.

Sitting down with Sturgess in a cushy hotel room on the outskirts of Bel-Air, it’s a far cry from the blistering conditions the film’s cast and crew endured in the Moroccan desert after a bone-chilling stint in Bulgaria. The British actor, wearing baggy jeans, a heather grey T-shirt and worn leather bomber jacket, has a charming ebullience as he discusses his role as Janusz, a young Polish inmate who leads the group on their grueling trek, an experience that was moderately echoed in the film’s production. Shot chronologically, “which is such a huge blessing with a film like this,” the actor says, Weir stripped the production down, which meant none of the usual luxuries actors experience when working on a film, such as air-conditioned trailers or overflowing craft service tables, which was helpful considering the actors were meant to look emaciated and near starvation.

At the start of each day, the actors would pull on their costumes and go through hair and makeup before piling into vans, which would take them to the set.

“We’d have to drive on fairly off-terrain places, often an hour, two hours, into the wilderness. They’d put up a tent or there’d be some place to sit down, but we were stuck in the same place for the entire day,” Sturgess describes.

Presented with a production schedule that would mimic the escaped prisoners’ journey as closely as possible, Sturgess didn’t feel nervous or put upon; he met the trial with enthusiasm.

“Everything that seemed like a big challenge seemed really exciting. That was the pull, the draw, to do a film like this,” he says. “I knew it would be an epic sort of adventure, on and off the camera. We were all going to have this life experience just by the pure nature of what we were about to take on. I knew I’d be freezing cold. I didn’t quite account for how hot I would be in the desert,” he admits with a laugh. “I’d never been to the desert before. I didn’t know how dry and unbearable it can get out there, and there’s nowhere to hide. That was hard.”

Despite the taxing nature of the endeavor, Sturgess says he was thrilled to be starring in the film and charged forward because of one simple reason.

“‘Peter Weir’ was all I had in my mind,” Sturgess says with a massive grin. “That he was going to make a great film in the end of it, that it [was] all going to be worth it, which is true.”

In order to prepare for the role, Sturgess says gamely, “The first thing I thought was, ‘Right, OK, I need to starve myself.’ I spend a day without food, just to see what that felt like, just because I was curious. It was interesting just not to eat or drink for a day.”

In the midst of his preparation, the actor went to Bulgaria, which stood in for Siberia, to meet with Weir.

“He took one look at me and was like, ‘What have you done to yourself? You have to put some weight back on yourself,’” he says with a chuckle. “Which made sense because we had to go through this journey. You had to believe Janusz would survive this. I already looked like I was about to keel over.”

To assist his actors, Weir brought in a French expeditionist, who, Sturgess enthuses, “had done the walk for real, from Siberia to India, having heard this story. He was an amazing man to have around.”

Questioned as to why anyone would tackle such an endeavor without the necessity of knowing prison guards are after you and it’s a matter of life or death, Sturgess beams and says with a glint in his eye, “Because he’s crazy. For fun. Because that’s what you do when you’re an expeditionist, I guess. He did a whole slide presentation, photos he’d taken, how it felt, how your balance would be off if you hadn’t eaten enough or your vision would be impaired. We’d go on treks and camping expeditions; make our own fires, build traps, we skinned our own rabbits. It was a real bonding experience.”

In another essential part of research, Sturgess says, “I was able to meet people who had survived the camps and an actual escapee. That was invaluable, to look at him and realize he isn’t any stronger than anybody else physically. He’s not a particularly big man, not tough, outdoorsy. He’s just a really average, regular guy, but had a reason, a will to get back. It’s hard to know what you could survive and what you would endure, how strong your spirit really is. I don’t know that you’ll ever know until you’re faced with that kind of situation. It was incredible.”

When in doubt during filming, Sturgess turned to Weir and found a wealth of knowledge from the compulsively prepared director.

“You just know you’re in such good hands, and you know whatever he says is probably the right answer. He’s so hungry for all information coming from any direction. I honestly believe if someone whose job was to make the tea had a suggestion, he’d be like, ‘Tell me about that. Please! I want to hear about it.’ He’s so thorough and so kind and so open, and he’s so caring and he’s so brilliant, you feel so safe. And he’s so detailed!”

To give an example, Sturgess recalls a day on set when he asked a reasonably simple question.

“He was right in the middle of filming, he had a lot on his plate, and he said, 'Gee, Jim, I don’t know the answer to that. Let me get back to you, and I’ll let you know.’ In the morning, there was a book with pink Post-it notes all the way through the book related to the question I’d asked him.”

Despite everyone’s passion for the project and Sturgess’ gung-ho-ness, once on set the extremity of the story and the filming conditions required to capture it faithfully began to take a serious toll on the cast and crew. But, just as in the script, the arrival of a young woman proved to be the salve everyone needed.

Saoirse Ronan, who’s been consistently impressive since her American debut in 2007’s Atonement, a film that marked her fourth role and earned her Oscar and Golden Globe nominations, arrived on set in the middle of shooting and was “a breath of fresh air!” Sturgess sighs. “It was welcomed in every way. Saoirse is such a great person. She’s hilarious, feisty and full of life.”

After months in the company of men, with their heads down, totally engrossed in recreating a hellacious experience, Ronan, who plays Irena, an orphaned refugee who falls in with the group of escapees, reinvigorated the production.

“We’d all done all this research, and we’d been shooting up at the gulag, super intense, and then suddenly this youthful spirit [arrived]. I remember looking at her and thinking, ‘Wow, you look so clean! So fresh. I just want to hug you.’ She was in real life the way she was in the film. I honestly don’t know if I would have survived or had the same experience had she not been there.” Sturgess explains that Ronan, who was only 14 during the shoot, became their “reason to be able to have fun. She needed to be entertained, and there was a selfish compassion that by entertaining her you’re allowing yourself [to relax]. The spirit of youth and the female spirit was a necessary one. It gave you extra life, and life was the most important thing we had.”

The Way Back releases in select theaters Jan. 21.