What began as a "chipper" day on set quickly went downhill as they filmed a nude brawl for the upcoming film.
In the film, adapted from the autobiographical book and 1977 film of the same name, the two play prisoners in a French Guiana penal colony who form an unlikely bond while trying to escape.
One of their many obstacles includes a group of assassins who try to kill them at their most vulnerable: naked, in the shower.
"It was a pretty brutal day," Malek told TooFab about shooting the scene.
"Two days, separated by a month I believe," Hunnam corrected. "Remember, the first day we shot it, we were actually in really good spirits and sort of just having fun with it. The water was cold, but actually the environment, it was quite a warm day."
"When we came back to shoot the second half of it, it was a month later, and we were into real Montenegro winter and it was very very cold and our mood was significantly less chipper," he continued.
Not only were they exposed during filming, but they were also transforming their bodies as the shoot went on appear as if they'd been on the island for a long time. It wasn't exactly a fun combination.
"It's hard to be really, really cold all day. We were both sort of starving ourselves, we were malnourished, just getting slammed around," Hunnam continued. "There's no opportunity for any padding or anything when you're naked, so when you're getting slammed down onto the hard floor."
He added, "It was one of those days where you think, 'I don't often feel like I'm earning my money, but I am today.'"
Not only do their characters have each other's backs during the fight, but the actors did on the set too when it came to losing weight.
"It's hard enough on any film set trying to keep yourself in the right headspace in-between takes, those hours can wear on you sometimes," said Malek. "You add in not eating, being hungry all day, you definitely want to go through that with someone else. As difficult as it was, it brought us together."
Hunnam ended up losing 40 pounds for the role of Henri "Papillon" Charrière, a real life safecracker who wound up in prion after being framed for murder. Through his time on "Devil's Island," Charrière found himself in solitary confinement, pushed to the brink of death with starvation.
While the actor also had a major weight transformation for his work in "The Lost City of Z," he learned something about his body from that experience that he made sure to remember when filming wrapped this time around.
"I did this twice in a row, I lost a lot of weight for the film I did right before this and I went out and had a blowout meal the night that I wrapped," Hunnam explained. "I had a big steak, french fries and ice cream and everything and was really sick for about 24 hours after that."
"Not actually vomiting but my body just went into complete shock," he continued. "So I didn't this time."
For Hunnam, the hardest transition was getting out of that mental headspace when all was said and done.
"The toughest thing in my experience in making films is attempting to re-integrate, the re-entry into one's normal life after you've had this experience that's so immersive and nourishing and creative fulfilling and gives one this singular purpose for everyday and it ends and all of a sudden you have this enormous absence in your life," he explained.
"There's a lot of neglect that's happened to your personal life when you've been in this experience, so life needs to be rebooted a little bit," Hunnam continued. "It's always difficult. On this, my girlfriend said, 'Just go off and escape and do what you need to do for the film, but don't come home until you're ready to see me.'"
Hunnam said he actually checked into a London hotel to "decompress" for about a week, where he learned "how to eat again" and "try to remember how to be a human being."
"Papillon" hits theaters August 24.