The recent run of World War I centennial anniversaries led to a spike in interest in the conflict, which ended in 1918, and Hollywood has been no exception. The few critically acclaimed Great War movies, such as All Quiet on the Western Front (1930) and Sergeant York (1941), were joined in 2018 by Peter Jackson’s documentary They Shall Not Grow Old. On Christmas Day, that list will get a new addition, in the form of Sam Mendes’ new film 1917.

The main characters are not based on real individuals, but real people and events inspired the movie, which takes place on the day of April 6, 1917. Here’s how the filmmakers strove for accuracy in the filming and what to know about the real World War I history that surrounded the story.

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The real man who inspired the film

The 1917 script, written by Mendes and Krysty Wilson-Cairns, is inspired by “fragments” of stories from Mendes’ grandfather, who served as a “runner” — a messenger for the British on the Western Front. But the film is not about actual events that happened to Lance Corporal Alfred H. Mendes, a 5-ft.-4-inch 19-year-old who’d enlisted in the British Army earlier that year and later told his grandson stories of being gassed and wounded while sprinting across “No Man’s Land,” the territory between the German and Allied trenches.

In the film, General Erinmore (Colin Firth) orders two lance corporals, Blake (Dean-Charles Chapman) and Schofield (George MacKay), to make the dangerous trek across No Man’s Land to deliver a handwritten note to a commanding officer Colonel Mackenzie (Benedict Cumberbatch), ordering them to cancel a planned attack on Germans who have retreated to the Hindenburg Line in northern France.

Life in the trenches

The filmmakers shot the film in southwestern England, where they dug about 2,500 feet of trenches — a defining characteristic of the war’s Western Front — for the set.

Paul Biddiss, the British Army veteran who served as the film’s military technical advisor and happens to have three relatives who served in World War I, taught the actors about proper techniques for salutes and handling weapons. He also used military instruction manuals from the era to create boot camps meant to give soldiers the real feeling of what it was like to serve, and read about life in the trenches in books like Max Arthur’s Lest We Forget: Forgotten Voices from 1914-1945, Richard van Emden’s The Last Fighting Tommy: The Life of Harry Patch, Last Veteran of the Trenches, 1898-2009 (written with Patch) and The Soldier’s War: The Great War through Veterans’ Eyes.

He put the extras to work, giving each one of about three dozen tasks that were part of soldiers’ daily routines. Some attended to health issues, such as foot inspections and using a candle to kill lice, while some did trench maintenance, such as filling sandbags. Leisure activities included playing checkers or chess, using buttons as game pieces. There was a lot of waiting around, and Biddiss wanted the extras to capture the looks of “complete boredom.”

The real messengers of WWI

The film’s plot centers on the two messengers sprinting across No Man’s Land to deliver a message, and that’s where the creative license comes in. In reality, such an order would have been too dangerous to assign.

When runners were deployed, the risk of death by German sniper fire was so high that they were sent out in pairs. If something happened to one of them, then the other could finish the job. “In some places, No Man’s Land was as close as 15 yards, in others it was a mile away,” says Doran Cart, Senior Curator at the National World War I Museum and Memorial in Kansas City. The muddy terrain was littered with dead animals, dead humans, barbed wires and wreckage from exploding shells—scarcely any grass or trees in sight. “By 1917, you didn’t get out of your trench and go across No Man’s Land. Fire from artillery, machine guns and poison gas was too heavy; no one individual was going to get up and run across No Man’s Land and try to take the enemy.”

Human messengers like Blake and Schofield were only deployed in desperate situations, according to Cart. Messenger pigeons, signal lamps and flags, made up most of the battlefield communications. There was also a trench telephone for communications.

“Most people understand that World War I is about trench warfare, but they don’t know that there was more than one trench,” says Cart. “There was the front-line trench, where front-line troops would attack from or defend from; then behind that, kind of a holding line where they brought supplies up, troops waiting to go to to the front-line trench.” The “bathroom” was in the latrine trench.

There were about 35,000 miles of trenches on the Western Front, all zigzagging, and the Western Front itself was 430 miles long, extending from the English Channel in the North to the Swiss Alps in the South.

April 6, 1917

The story of 1917 takes place on April 6, and it’s partly inspired by events that had just ended on April 5. From Feb. 23 to April 5 of that year, the Germans were moving their troops to the Hindenburg Line and roughly along the Aisne River, around a 27-mile area from Arras to Bapaume, France. The significance of that move depends on whether you’re reading German or Allied accounts. The Germans saw it as an “adjustment” and “simply moving needed resources to the best location,” while the Allies call the Germans’ actions a “retreat” or “withdrawal,” according to Cart.

In either case, a whole new phase of the war was about to begin, for a different reason: the Americans entered the war on April 6, 1917. A few days later, the Canadians captured Vimy Ridge, in a battle seen to mark “the birth of a nation” for Canada, as one of their generals put it. Further East, the Russian Revolution was also ramping up.

As Matthew Naylor, President and CEO of the National World War I Museum and Memorial in Kansas City, Mo., says of the state of affairs on the Western Front in April 1917, “Casualties on both sides are massive and there is no end in sight.”

Correction, Dec. 24

The original version of this article misstated how WWI soldiers de-loused themselves. The troops used a candle to burn and pop lice, they did not pour hot wax on themselves.

Write to Olivia B. Waxman at olivia.waxman@time.com.