Badass "Sons of Anarchy" star Charlie Hunnam is taking quite a departure in his new movie "The Lost City of Z." As Percy Fawcett, he plays an entirely different character than Jax Teller in SOA. And it shows in his shocking weight loss. What's up with the Geordie actor? Co-star Robert Pattinson spilled his secret and it may shock you as much as the true story behind the their new film.

Charlie Hunnam swaps anarchy for anacondas

The "Sons of Anarchy" star traded in his tats and leather for a muttonchops and a WWI uniform to play the British archaeologist Col.

Percival Fawcett. Hunnam dismounted the iron horse to take up the machete in this 1920s story of adventure and lost civilizations. The historical biopic, co-starring Sienna Miller, follows the expedition into the Amazon rainforest to find an ancient lost city "Z" (pronounced "zed") believed to be El Dorado. Leader Percy Fawcett never returned and his disappearance remains a mystery to the present.

Charlie Hunnam's mysterious look after "Sons of Anarchy"

The whereabouts of the intrepid colonel isn't the only mystery. In the "The Lost City of Z" Hunnam is almost unrecognizable in the role. In fact, "Sons of Anarchy" fans didn't believe it was him. As FX's motorcycle clubbie Jackson Teller, Hunnam was beefed up.

After his Hell's Angels character, he appears anorexic as the slender, wiry soldier in "Lost City of Z." Fawcett has the same chutzpah as the Harley rider, but his battle isn't with rival gangs. The fight is to get the Royal Geological Society to support an expedition into the Amazon. The movie isn't Hunnam's first role in a history drama.

He played Nicholas Nickleby in Charles Dickens' eponymous story.

Charlie Hunnam lost how much weight and why?

To play the bespectacled cartographer, Hunnam had to lose weight. "Twilight" actor Robert Pattinson, who plays Henry Costin in "The Lost City of Z," said they were each forced to shed 35 pounds. He and Hunnam agreed that it was grueling.

Both were irritable from their "eating disorder" diet. But living starvation conditions was essential to make them convincing. Liam Neeson and other actors in the Martin Scorsese film "Silence" had to shed 50 pounds each. That kind of weight loss is usually only advised for people with morbid obesity.

Charlie Hunnam yo-yo diets for "King Arthur"

After "The Lost City of Z," Hunnam played the titular role in "King Arthur" and had to gain weight for the part. This is a common requirement of directors. "Star Wars" actor Adam Driver (who played Kylo-Ren) recalled that he and Andrew Garfield starved themselves as if they had an eating disorder for "Silence." All three went on to play roles of normal weight after portraying Jesuit priests taken prisoner. So yo-yo dieting is common in Hollywood but is it safe?