"We both managed to find a way to fall asleep on demand," Fionn Whitehead says of catching impromptu zzzs with costar Harry Styles

By Kara Warner
July 19, 2017 06:53 PM

In addition to developing a new appreciation for the real-life soldiers who fought and survived at Dunkirk, the cast of the Christopher Nolan-directed WWII epic inspired by the historic feat picked up several practical skills during filming — chief among them, an ability to fall asleep anywhere.

The film’s young stars Fionn Whitehead, Harry Styles and Jack Lowden recently shared with PEOPLE their favorite moments on set, advice they received from their more experienced costars, as well as a newfound respect for hot showers. Read on to learn five set secrets from the cast.

Fionn Whitehead and Harry Styles initially bonded over a highlighter

“We met in the audition process,” Whitehead recalls. “The final audition I did, I was screen testing with Harry and another actor and I think needed a highlighter or something [to highlight lines in the script] and I went, ‘I could really use a highlighter.’ And Harry said, ‘Need a highlighter?’ and just disappeared upstairs and got me a highlighter, which was very nice of him. Such a nice young man.”

Harry Styles is “the best” at falling asleep anywhere

“I think we both managed to find a way to fall asleep on demand,” Whitehead says of scoring impromptu rest during downtime. “So, on set if there was a five minute break, we just turn over and zzz. [Turns to Styles] I think you’re the best at it, actually. I like to think I’m pretty good, but I think you’re the best at falling asleep.”

The two actors even scored some shut-eye while filming scenes in the ocean.

“I definitely fell asleep while floating a couple times,” Styles admits. “It was nice, refreshing.”

When they weren’t filming, they were freezing

“There were a couple days when we were just out in the water all day that felt like they were the most rewarding days when they were done,” says Styles. “During they were the worst days, but as soon you get home and shower it was the most rewarding part of the thing.”

Whitehead adds: “I find it really funny that they’d kept running in with warm coats, but we were covered with water. So, they’d give us these warm coats and then we’d get the inside wet and then we’d put them back on and they’d be all slimy.”

For much more on Harry Styles and Fionn Whitehead, pick up the latest issue of PEOPLE.

FROM PEN: 23 Oscar Nominees Reveal The Untold Stories Behind Their Films

Tom Hardy gives great career advice

“He has a lot of time for young actor,” reveals Scottish newcomer Jack Lowden, who plays a Spitfire pilot alongside Hardy’s character. “I will be forever in debt to him for some of the advice he gave me. He’s a very smart man and he understands what it’s like when you’re starting out. ‘Make it just for you,’ he sort of said it just like that. And said to play the part like your life depends on it. I spent a whole week with him, he’s an absolute sweetheart.”

The actors did not earn their pilot’s licenses, the production used real pilots to fly the WWII-era planes called Spitfires

“I wouldn’t say that I know how to fly a Spitfire, we had some brilliant pilots doing that, but I certainly know what it was like,” explains Lowden. “It’s really intense, claustrophobic and hot. It must have been exhausting. It always amazes me how young these guys were. I think the youngest was 18 and you start thinking, ‘What was I doing at 18?'”

Experience Dunkirk in virtual reality with the LIFE VR app! Download the app for iOS and Android or visit See a 360° preview here.

Dunkirk opens in U.S. theaters on July 21.