"I felt as though it was going to require more from me than I'd ever had to give," Hunnam, 37, said of playing the role of Percy Fawcett in the film that is based on the true adventures of the Amazonian explorer, who went missing in 1925 while on an expedition to find an ancient lost city.
Hunnam told ABC News that he decided to truly get into character and attempt to understand his character's feelings of loneliness and isolation by cutting himself off from all outside communication while shooting the film in Colombia.
"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," Hunnam said of his character.
"I'm lucky that I have a very understanding girlfriend who allows me to go off ... and do these weird whims." " Hunnam added.
While in Colombia, Hunnam did have the company of a bearded Pattinson, who plays Fawcett's cohort Henry Costin.
Pattinson said that being cut off from the outside world was "incredible."
"'I'm sorry I have to go for six months and I am deeply in character. Don’t call me,'" Pattinson joked.
Hunnam added that he "didn't use email once" during the shoot.
"I didn't use email once. I didn't go on the computer. I didn't make a phone call for four months," Hunnam said. "It was so liberating."
In addition to giving up communication, Hunnam also said that the role required him to depict some of the hardships that Fawcett faced while on his expedition and drop more than 30 pounds.
"I don't think I was ... too much fun to be around," Hunnam said, adding that there were "sort of suspicious glances" whenever he went off set as some of the crew speculated that he was going to go eat something.
Hunnam added, however, that he was drawn to the sense of adventure in the film.
"I totally got that obsession that Fawcett had," he said, "That, 'Let's just go a little further and a little further,' and 'What's over the next hill?'"
"There were moments where me and Charlie were just kind of hacking through jungle with machetes," Pattinson added. "And it's virgin jungle, it's in the middle of the jungle, and all the Colombians, they were like, 'What are you doing? Stay on the path ... you're gonna get bitten by something.'"
"The Lost City of Z" will be released in theaters nationwide on Friday.