There’s an astonishing shot at the climax of the new war film 1917.

British actor George MacKay is seen sprinting down the frontlines, dodging crossfire and charging soldiers like a terrifying gauntlet. His eyes are intensely focused ahead of him, as if determination had manifested itself as an engine propelling him along, rendering him almost oblivious as bullets whiz past and explosives detonate behind him, backdropping his dash with rising smoke.

The stirring scene is filmed to appear as if it’s part of the movie’s one continuous take, the groundbreaking style in which director Sam Mendes and legendary cinematographer Roger Deakins shot the World War I epic. To pull it off, a camera operator had to begin by sprinting alongside MacKay, then hook the camera onto a wire that would fly it across more land, after which it would be unhooked again and run with an operator to small Jeep, which would then carry the camera about another 400 yards to the end of the take.

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When the scene plays at screenings, the entire audience is typically found leaning forward in their seats, their hands to their mouths, eyes riveted to the screen. As breakout moments for the leading man in a massive war-movie production go, they don’t come much more jaw-dropping than this.

MacKay’s eyes widen when I mention the shot. Meeting at SoHo’s Crosby Hotel in Manhattan a few weeks before 1917’s Christmas Day release, he lets out a breath so deep that it seems like it’s been held in since finishing up that monumental run on set. “That,” he says, punctuating the word, “was a big day.”

“When I first watched it, I got really emotional,” he continues. “I don’t mean in a sort of wanky way, like talking about watching myself. But with the character, there’s such purity to someone really going for it like that.” Even now, he starts to get a little teary again, something that makes him sheepishly laugh at himself. After a sigh: “That was a tough run.”

Truthfully, it was one of many tough runs.

Because 1917 sets out to viscerally attach the audience to the journey of a British soldier through an unforgiving, deadly stretch of World War I, the camera never stops following MacKay. For the audience, it’s an immense, anxiety-inducing experience. It’s like being alongside a video-game character as he graduates each level and has to face off against a more terrifying, unbeatable boss. Only it’s a human. There are real lives at stake.

Reactions from critics and industry members at early screenings called the film “a stunning cinematic achievement,” complimented the one-shot technique for rising above gimmickry and “putting us right there in this hell,” and hailing it as “the best war film since Saving Private Ryan”—though there are plenty who would argue that “since The Thin Red Line” might be more appropriate, with a few shout outs for Dunkirk from the back.

About a month into filming, MacKay had a realization: It felt like he had been running for three weeks straight. He chuckles remembering what Mendes said to him in his trailer on the last day of production: “You’re going to have to level with the fact that you probably will never be as fit again in your life.”

1917 is fact-based fiction about a significant turning point for the British and Allied forces in World War I. Lance Corporal Schofield (MacKay) and Lance Corporal Blake (Game of Thrones’ Dean-Charles Chapman) are two young men unwittingly assigned an impossible mission: traverse through the heart of enemy territory to deliver a message that, if it arrives on time, would save the lives of 1,600 soldiers. Among those men is Blake’s brother.

Mendes co-wrote the screenplay with collaborator Krysty Wilson-Cairns, inspired by the stories his grandfather, Alfred H. Mendes, told him.

He was a Lance Corporal in the First World War, enlisting in 1917 when he was 19 years old. Not even five-and-a-half feet tall, he was drafted to be a messenger, capable of sprinting undetected through the mist that rested on No Man’s Land, the unclaimed land between Allied and enemy territory that no one traversed for risk of being killed, and delivering messages from post to post.

Mendes pieced together his grandfather’s anecdotes with other accounts from veterans he discovered over the years, inspiring the idea of a simple story—one young man carrying a message from one place to another—that becomes epic in scope when the stakes are considered.

“These stories personalize the massive loss of life and the sacrifice,” MacKay says. “We learn about it at school, God, tens of millions lost. But to understand Schofield and to value him as a person, and to think of Sam directing us and being a descendant of Alfred, you go, God, every one of those men either came from a family or had their own or both. You don't just lose bodies, you lose personalities, you lose friends, you lose brothers, you lose husbands. That's when it really hits home.”

It’s not a slight to MacKay or his co-star, Chapman, that it was important for these soldiers to, at first glance, seem unremarkable.

“ He chuckles remembering what Mendes said to him in his trailer on the last day of production: ‘You’re going to have to level with the fact that you probably will never be as fit again in your life.’ ”

Casting a recognizable A-list celebrity would be a disservice to the idea that these were two ordinary men no different from any of the young soldiers who surrounded them, outside of the fact that they were the two chosen for the mission. It’s only in brief, one-scene appearances that a handful of Britain’s more famous stars—Colin Firth, Benedict Cumberbatch, Richard Madden, Mark Strong, and Andrew Scott—appear along the two boys’ journey.

Growing up in London, MacKay’s first foray into film came when an acting scout asked him if he would audition for P.J. Hogan’s 2003 adaptation of Peter Pan. He was cast as one of the Lost Boys, which had him cavorting around the Queensland forest for eight months where the film shot in Australia.

He quickly accrued a slew of notable film and TV roles, and his profile has risen significantly in recent years. He played a young gay man coming of age in 2014’s Golden Globe-nominated Pride, and the eldest son of Viggo Mortensen’s stubborn survivalist in 2016’s Captain Fantastic—for which he learned how to track animals, start a fire from scratch, and master a spiritual form of yoga.

With wide, dark eyes dancing from an angular, handsome face, a slight frame, and unassuming demeanor, there was something old-fashioned about MacKay’s look that appealed to Mendes. He wouldn’t just pass as a British soldier in 1917, but he also looked like someone capable of mustering the dignity and the fortitude to devote himself to the death-defying task at hand.

But what MacKay accomplishes is much more than your blank slate, Everyman tour guide through the atrocities of war. It’s an impressively nimble performance, somehow telegraphing a kaleidoscope of emotion—fear, desperation, grit, hopelessness, hope—at shutter speed while isolating each one at the extremity it warrants.

In person, the 27-year-old is the platonic ideal of the charming young British man: humble, charismatic, polite, constantly tousingly his stylishly messy hair.

“I feel quite close to Schofield as a character,” he says. Not that he thinks he could pull off the mission. But, albeit in different contexts, their way of operating is the same.

“The story starts on the move and ends on the move,” he says. “One of the biggest life lessons I took from this is that life just keeps on moving, you know? The camera sort of pushes and pulls you through the film, through all the obstacles that have to be overcome, slowly but relentlessly. In hindsight, those things become a milestone. But in the moment, this is just another day.”

MacKay, Mendes, and a swath of the cast and crew began rehearsing the film last winter. Because of the logistics of the one-shot technique, those planning sessions ran longer than most films, if any, have the opportunity for. Filming only began officially in April and wrapped three months later in July—a remarkable, essentially unheard of turnaround for a film on this scale.

Once filming started it was like the showbiz cliché of a big production number with chorus boys spinning plates and acrobats flying trapeze while pyrotechnics explode. No matter how famous the performer is who is leading the number, if they go out on the lyrics, the whole event comes crashing down. “If your rifle slips or something, it’s back to one,” MacKay says. “It was brilliantly unrelenting.”

To keep continuity, Deakins and the filmmaking team relied on the light. That meant having to wait for cloudy, gray skies to shoot any footage, maybe the only time a group of Brits stared at the sky praying for the sunshine to go away. It also meant long hours spent shadowed in darkness in wet mud, trudging through trenches and across freezing, water-logged battlefields.

“Sam wanted us to live it as much as possible,” MacKay says. “But you realize it's a drop in the ocean compared to the real thing. You go, Oh God, I'm wet or I'm tired and filthy. But I can count the hours on one hand by the time I get out with these clothes.”

He gives one of those timeless, young British-boy sly smiles that Mendes was so attracted to when he was casting. The minutiae of filming is endlessly fascinating to him, but he can’t tell how much of the technicality would be interesting to others—let alone the grander existential thoughts he had while immersed in the process. (Suffice it to say it’s all interesting.)

The insecurity is understandable, though. By the time Christmas rolls around, the movie hits theaters, and the award season he’s going to be promoting in earnest come January, MacKay will have spent the entirety of 2019 in the mindset of a soldier more than a century ago. “This whole year has been 1917.”