Robert Pattinson and Charlie Hunnam
Charlie Hunnam (L) and Robert Pattinson (R) attend premiere of Amazon Studios' 'The Lost City Of Z' at ArcLight Hollywood on April 5, 2017 in Hollywood, California Rich Fury/Getty Images

Robert Pattinson had to undergo a transformation while filming upcoming movie The Lost City Of Z. Not only did he shed his teenage heartthrob image to take on the more serious role of Amazonian explorer Henry Costin, but he also dropped around 35lbs for his character.

The same applies for co-star Charlie Hunnam, who admitted to have lost a fair amount of weight and aswell as isolate himself from the world as he prepared to portray the real-life experiences of British explorer Lieutenant Colonel Percival Fawcett.

"I felt as though it was going to require more from me than I'd ever had to give," Hunnam said about his character, who went missing in 1925 while searching of a secret ancient city.

Both Pattinson and his co-star revealed the extreme steps they took as they opened up about filming the historic adventure on Good Morning America.

Offering an insight into the life of his character, the Sons Of Anarchy star said, "He would go on these grand, heroic explorations into the jungle for three or four years at a time, and have no contact with his family at home."

"I'm lucky that I have a very understanding girlfriend who allows me to go off... and do these weird whims," he added.

During the adventurous experience of filming in the jungles in Colombia, Hunnam was disconnected from the world. His only company was his co-star Pattinson.

"It's incredible," the Twilight star said, about going off the grid. However, Pattinson might have sunk in too deep into his character as he joked about telling friends and family: "I'm sorry, I have to go for six months and I am deeply in character. Don't call me."

Hunnam agreed on the "liberating" experience of staying away from the limelight and being cut off from technology. "I didn't use email once. I didn't go on the computer. I didn't make a phone call for four months," he said. "It was so liberating."

"I totally got that obsession that Fawcett had...'Let's just go a little further and a little further', and 'What's over the next hill'?" Hunnam said, of the experience.

Pattinson also spoke about moments when they would go "hacking through [the] jungle with machetes".

"It's virgin jungle... and all the Colombians, they were like, 'What are you doing? Stay on the path, you're gonna get bitten by something'," he said.