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It’s not just any mud. In 1917, the muck and mire of “No Man’s Land” traversed by two British soldiers on a mission to save 1,600 fellow soldiers from a German army ambush, oozes the very essence of messy wartime chaos. But like everything else in Sam Mendes’s World War I movie, even the mud was fine-tuned to the director’s exacting standards.

Production designer Dennis Gassner mixed in soil samples from outside the battlefield location to achieve the perfect shade of brown.

“The battlefield in 1917 was like doing a giant 3D painting of emotion,” Gassner says. “We had to create a landscape that tells you what these soldiers are feeling. You do that with two things that, to me, are paramount in designing a film: architecture and color.”

1917 (2019) — Production designer Dennis Gassner on the set of Sam Mendes’s World War I epic, “1917.”

Gassner, an Oscar nominee for Blade Runner 2049, collaborated previously with Mendes on four movies including Gulf War movie Jarhead and the last two James Bond pictures. Speaking to Fortune recently from Maui, where he’d been resting up from the shoot, Gassner says he approached 1917 using the system he learned early in his career as an assistant to Apocalypse Now production designer Dean Tavoularis.

“I lay the whole thing out on the walls my office,” he says. “Those plans are scrutinized and modified to fit the emotional structure of the film from scene to scene. Then we build. When we see the physical reality, there are always adjustments and adjustments and more adjustments, constantly looping back to the director’s vision until everything connects as one.”

Timing Is Everything

But with 1917, Gassner faced an unprecedented challenge: Mendes envisioned a low-to-the-ground tracking shot that appears to follow the heroes through their entire journey in one continuous take, disrupted only by a brief blackout. In a director’s statement, Mendes said: “I wanted to travel every step and breathe every breath with these boys… Our camera never leaves them. We designed 1917 to bring audiences as close as possible to their experience.”

1917 looks as if Oscar-winning cinematographer Roger Deakins shot the entire story within 24 hours at a single location. In fact, principal production lasted about 65 days and extended from Salisbury Plain in southwest England to the Govan docks in Scotland, the River Tees in North England, a disused quarry in Oxfordshire, and the Bovingdon Airfield outside of London. Tasked with creating a seamless blend of continuous action, Gassner says, “The whole idea was not to get caught. Sam challenged everyone to figure out how to make connections [between shots] so we could arrive at the next location, do the cut and make sure there was no artifice in the lighting or the architectural situation.It all came down to tireless planning. Tireless. Down to the inch. Seriously, to the inch and down to the second. Sam would not let it go until we had each sequence that precise. Once our design had that [precision], then we could do the work.”

Early in 2019, Mendes, drawing on his background as a theater director, blocked out the action on a London soundstage using cardboard boxes as stand-ins for ruined farmhouses and sniper positions. His team then traveled to southwestern England. “We rehearsed in fields and snow and rain and bitter winds,” Gassner recalls. “It was very harsh work.”

The Trenches

The film begins and ends in the trenches, with more than 5,200 feet of period-perfect ditches designed to accommodate walk-and-talk scenes between stars George MacKay and Dean-Charles Chapman. “Sam, Roger and I walked the trenches, measuring it to the dialogue,” Gassner says. “It was all about accuracy, with Roger and Sam basically choreographing the movement. I’ve worked with Roger for 38 years, Sam for 19. We figured out what had to be done, and then we did it because we all share this passion for getting it right.”

To construct the type of trenches that millions of soldiers called home during World War, Gassner pored over some 50,000 archival images and spent two weeks in France. There, he studied trenches and tunnels that had been restored at Vimy Ridge, site of a massive World War I battle. “We mimicked the way the British army would have built these trenches and the same with the Germans. The trenches were very reflective of the personalities of the countries that built them.”

German-engineered trenches were rectilinear, capacious, concrete-lined structures. British trenches had a more homespun look, lined with woven wheat straw used to make thatched roofs for traditional English cottages. “We sourced the same materials that existed back then and our art department built it the same way the soldiers themselves dug the trenches back in 1917,” Gassner says. “Same thing with costume designer Jacqueline Durran’s uniforms, which were made from mouton wool used by the British Army in World War I. It became heavy when wet. We wanted to show the experience of what these soldiers really had to go through.”

The Art of War

Mendes modeled 1917 on World War I stories told by his grandfather, an English soldier, who at 5 feet, 4 inches, was assigned to carry messages because he could travel unseen by enemy troops under cover of the five-and-a-half-foot mist that typically hovered over the battlefield. In honor of those who fought and suffered during what was once called “The Great War,” Gassner sought to impart a certain austere beauty to the bombed-out farmhouses, the ruined villages, the wrecked bridges, the cannons and craters, which were all built from scratch for the film.

“Working with Sam on Jarhead, I had to find a safe place to protect myself because we were dealing with such horrific things,” Gassner says. “That’s when I came up with this term for myself, ‘the art of war.’ You’re trying to protect the audience a little bit by letting the human story come through rather than just showing the atrocities. With 1917, I said to the art department ‘When in doubt, make it as beautiful as possible, even though we’re dealing with horrific things.’ It has to do with respect. Respect for the experience these soldiers had to go through.”

1917 is currently out in limited release with a wide release slated for Jan. 10.