To bring The Revenant's epic story of survival to life, Leonardo DiCaprio, along with the rest of the cast and crew, worked in conditions similar to the film's 1800s setting, the stars told ET in a roundtable discussion.
DiCaprio stars in The Revenant as famed, real-life frontiersman and explorer Hugh Glass, who gets viciously mauled by a bear while on an expedition of the uncharted American wilderness in 1823.
Director Alejandro Inarritu's obsession with authenticity meant not a single electric light was used on location. The actors were illuminated only by sunlight and fire, which meant time restraints were especially tight.
"We had to rehearse constantly to get these very meticulous shots out in nature perfectly set up and then we'd have an hour and a half of this natural light that we wanted to capture, so it was a mad scramble every day to try to capture everything within an hour-and-a-half time period," DiCaprio told ET. "I think all that tension and all that anxiety of having to accomplish that day infused itself into the movie as well."
Another authentic part of the movie was the aggressive facial hair.
"I was very proud of my beard," DiCaprio said. "It was a year and a half of living with that beard. It became like a very close friend, but I got rid of that friend as soon as I possibly could when the production stopped. I'm happy to have my beard gone."
One person who was suspiciously clean-shaven was 22-year-old Maze Runner star, Will Poulter, but he says this wasn't by choice.
"I'm being honest, that's seven months of no shaving!" Poulter joked.
Previously, DiCaprio has called The Revenant the most difficult shoot of his life.
"I can name 30 or 40 sequences that were some of the most difficult things I've ever had to do," the 41-year-old actor told Yahoo! Movies. "Whether it's going in and out of frozen rivers, or sleeping in animal carcasses, or what I ate on set."
The Revenant, which is already receiving a lot of Oscar buzz, comes out in limited release on Christmas Day, and opens wide Jan. 8.