When director Sam Mendes told frequent collaborator Roger Deakins he wanted to film his upcoming World War I drama “1917” to appear as one continuous take, the renowned cinematographer had just one question: “Why?”

It was a valid query, Mendes admits. “I want to step every step with the characters, I want to breathe every breath with them, I want to be trapped on their journey with them,” Mendes told a packed crowd at AMC 34th Street during a panel at New York Comic Con.

Mendes, joined on-stage by Deakins, co-writer Krysty Wilson-Cairns, producer Pippa Harris and co-stars George MacKay and Dean-Charles Chapman, also treated attendees to a behind-the-scenes feature and two trailers for “1917,” which Universal Pictures is debuting in theaters this December.

“1917” is set during the height of the First World War and unfolds in real time as two young British soldiers are tasked with delivering a message that will save the lives of 1,600 men. Mendes knew that Deakins’ involvement was essential in capturing the emotional experience of accompanying the soldiers on a gripping, edge-of-your-seat journey that’s made to feel like the audience is in the trenches with them. Mendes emphasized the slice-of-time movie unspools over the course of two hours in one unbroken shot, sparking applause and setting off a chorus of “oohs” and “wows” from the eager Manhattan crowd.

“What Roger manages to achieve is for that camera to become a third character,” Mendes explained. “If you’re looking at this movie and thinking, ‘it’s one shot,’ I think we will have failed. If you don’t care about [the characters], if you don’t go on that journey with them, then what’s the point? We were on the same page about that from the very beginning.”

Wilson-Cairns was more candid with her reaction to Mendes’ bold vision: “I thought you were bats— the whole time, but I love bats—, so I was into it,” she joked. “It was like an army of people trying to do the impossible and somehow succeeding.”

Being the man in charge, Mendes attests if he had concerns about whether or not he could pull it off, the cast or crew couldn’t know.

“As a director, you can’t let on at any point that you think this might have been a bad idea,” he said to laughter from the audience. “There was no day during shooting I thought that, but there were several times in prep when I had to stop and think, ‘I actually can’t get what I want out of this scene without conventional coverage. How do I reshape the scene in an order that every shot of it works in order?’”

Those concerns led to meticulous planning, writing and re-writing from Mendes and Wilson-Cairns. Everything had to be perfectly timed to the second to pull off a seamless look.

“It was so exhilarating and exciting when it worked that it kept you going for days at a time,” said Mendes, who calls the film “the most exciting job of my career.”

Before cameras started rolling, the actors endured intense preparation.

“We rehearsed this movie more than I’ve ever rehearsed any movie,” Mendes said. “You can’t jump through space and time. You have to measure the set to match the dialogue, and you have measure the distances.”

That level of precision wasn’t lost on the actors on set, even if they weren’t aware of just how detailed Mendes and crew were. Mendes recounted a day on set when actor Mark Strong shot a scene with MacKay that involves him walking past four trucks filled with men.

“He got to the end of the dolly and said, ‘That’s amazing. What a coincidence that the dialogue lasted just until [we hit our mark],'” Mendes recalled. “I said that’s not a f—ing coincidence, we’ve been rehearsing it for six months.”

Deakins likened shooting, some of which involved takes that lasted eight-and-a-half minutes, to performing at the ballet. “It was this really awesome challenge. Everything’s got to be in sync,” he said. “When you’re almost to the end of the shot you’re like, ‘I hope I don’t blow this.’ It was a real trip.”

Before executing his unique filming concept, Mendes landed on the idea for what he calls his “passion project” from stories his grandfather told him about fighting in the war. The characters in “1917,” however, aren’t based on real people. But what does ring true, Mendes said, is “the spirit of it and what those men went through — the selflessness.”

The on-screen mission becomes especially emotional for one of the soldiers, Schofield, portrayed by Chapman, the actor most recognizable to “Game of Thrones” enthusiasts as Tommen Baratheon. His character’s brother is among the 1,600 men whose lives are at risk of ambush. Chapman’s research became a similarly personal process. While prepping, he discovered a close connection as he read the book “The Western Front Diaries,” a collection of journal entries from soldiers.

“I actually found my great granddad had a diary entry in there,” he shared.

Mendes chimed in, “You never told me that!”

“Yeah, he was part of the calvary and he was shot and wounded and he survived laying in no man’s land for four days. After the war he survived and worked at a poppy factory,” Chapman said as nearly everyone in the audience gasped.

“It’s hard to follow that. I actually fought in World War I myself,” MacKay joked of his own preparation. “No, I felt very close to Schofield as a man. We did a huge about of personal research.”

Mendes hopes the audiences is equally moved by the film. “I hope they’re not capable of speech for 10 minutes,” he said.