EXCLUSIVE: Christopher Nolan Compares Casting Harry Styles in 'Dunkirk' to Making Heath Ledger the Joker

The celebrated filmmaker opened up to ET about his decision to cast the singer based on talent and his 'truthfulness' as a performer.

When Christopher Nolan cast Harry Styles as a lead in his upcoming World War II drama, Dunkirk, the decision caused a stir among some skeptics who questioned the singer's acting chops and derided his involvement as stunt casting. However, the acclaimed director says he knew from the start that he chose the right man for the role.

Speaking with ET's Carly Steel at a press junket promoting his latest epic, Nolan said he really had no idea who Styles was when he auditioned, adding that the former One Direction singer got the part based on the talent that he brought to the table.

"As a director, I have to trust my instincts, my ability to figure out who's the right guy for the part. I'm not too worried about baggage," Nolan, 46, explained. "I was new to Harry. I mean, I've heard his name from my kids, but I wasn't really familiar with him… What I was seeing [when he auditioned] was a very charismatic guy who clearly had a truthfulness and a subtlety in his ability to perform as a film actor."

Nolan -- who has been nominated for two Best Original Screenplay Oscars for penning Memento in 2002 and Inception in 2011, which also received a Best Picture nomination -- said that he's no stranger to casting against type.

WATCH: Harry Styles Makes Big Screen Debut in 'Dunkirk'

In fact, the celebrated filmmaker says one of the most acclaimed performances in his oeuvre came from an actor that faced a lot of skepticism.

"When I cast Heath Ledger as the Joker [in The Dark Knight], it raised a lot of eyebrows and caused a lot of comment," Nolan said. "I have to trust my instincts, and Harry was perfect for this part."

"What I'm hoping for, when people see the film, is I'm hopeful that they won't miss what he's done, because it's very subtle, very truthful and real," Nolan continued. "I wasn't giving him a sort of flashy thing to do. But it's really important what he does in terms of what it says, I think, about human nature and what people do in different situations, and I think he pulled it off with incredible grace and reality. And that as a director, that's what you're looking for."

Warner Bros. Pictures

WATCH: Harry Styles 'Felt Very Naked' After Cutting His Hair

In spite of Styles' devoted fan base, most of whom fell in love with him during his time with One Direction, Nolan said the star's legion of screaming fans didn't get in the way of filming, or impact Styles' involvement.

"We always have a secretive set anyways. So we're sort of used to shutting things off and really trying to create our own world. So, none of that really intruded on our creative process, which was terrific," Nolan shared, adding that Styles didn't let his celebrity outside of the set affect his ego either.

"Harry was very committed and determined to just being one of the guys and coming there and learning what he could from the most experienced people around him and diving into the role feet first," Nolan recounted. "I think he's done an incredible job on the film and I'm really excited to show it to people."

For Styles, however, the most important part of part of the process was working together with the cast and crew to do justice to the story.

WATCH: Everything to Know About Harry Styles' Role in 'Dunkirk'

"The film is so amazing and I think it tells such an important story," the singer-turned-actor told ET at the junket. "I think [it had] such an amazing list of actors working on it and I feel like we were all working towards the same thing."

"I don't feel like [filming] really allowed any time to think too much about specifically what I was doing or anything," he added. "I think the movie is amazing and I just feel very lucky to kind of be a part of it, really."

tells the true story of one of the most harrowing battles of World War II, in which Allied soldiers from Britain, Belgium, Canada, and France were evacuated from the harbor in Dunkirk, France, during Operation Dynamo, as Nazi forces surrounded them and moved in to attack. During the evacuation, civilian vessels were called on to support the evacuation, and many families risked life and limb to help in the dangerous military operation.

For Nolan, who is best known for helming the Dark Knight trilogy and the mind-bending sci-fi dramas Inception and Interstellar, this is the first time he's taking on a true story, and the filmmaker said it's one he's been close to for most of his life.

"Like most British people, I grew up with the story of Dunkirk. It's taught to us very early on," Nolan told ET. "But [in school] we get this sort of fairy tale, kind of mythic version, very simplified."

NEWS: Harry Styles Officially Goes Solo, Shares Single Art and Release Date Ahead of 'SNL' Performance

He explained that the experience that finally made him want to retell the story was when he and his wife rode in a small boat with their friend across the ocean near the beaches of Dunkirk, which were the same waters Allied forces contended with during the evacuation.

"It was very difficult, it felt very dangerous, very uncomfortable, and that was with nobody dropping bombs on us. We weren't heading into a war zone," Nolan recalled. "So you come out of that experience thinking, 'If it's that hard, how difficult must it have been [for them]? How extraordinary is it?' Civilians were willing to make that trip to help out the guys who were trapped there on the beach. It's really one of the most remarkable stories in human history."

sails into theaters
July 21.

For more details on Styles' role in the upcoming war drama, check out the video below.