EXCLUSIVE: Charlie Hunnam Talks Packing on 20 Pounds of Muscle for 'King Arthur,' Fighting for the Role

Charlie Hunnam may seem like the perfect choice to play King Arthur, but the 37-year-old actor actually had to fight for the role.

ET's Lauren Zima caught up with Hunnam at the junket for King Arthur: Legend of the Sword in New York City on Sunday, where he revealed what it took to convince director Guy Ritchie that he was right for the part.

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"I'd lost an enormous amount of weight for the last season of Sons [of Anarchy], so I showed up very, very skinny, and he wanted someone who was sort of physically imposing for the role," the actor explained.

"There were three other actors auditioning for the role that day, big, well-known actors, and I said, 'Why don't you just bring them all in here and we'll fight, and whoever walks out the room, gets the role?' and that was the moment he sort of liked that moxie. I saw a little twinkle in his eye," Hunnam shared. "He said, 'Alright.'"

Michael Fassbender and Henry Cavill -- both of whom were in superhero shape -- were reportedly also up for the role. But Hunnam gave his all to transforming into King Arthur, packing on 20 pounds of muscle.

"The hardest part is always maintaining the diet, because you just do not get to eat all of the fun things you want to eat. And also eating an enormous amount because this is not my neutral body size," the actor confessed. "I was probably 20 pounds heavier than my body naturally wants to be, so it is a full-time job to keep that muscle on."

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"I did an enormous amount of pull-ups," he added. "Pull-ups were like the staple, pull-ups and dips."

Someone not too impressed by his new physique, however, was Hunnam's girlfriend, jewelry designer Morgana McNelis.

"She thinks it is a bit silly," he admitted. "She says it makes her feel really small... I think she likes it in that regard, but we have been together for so long, it's like, 'Put your muscles away and come to bed.'"

After 11 years together, however, Hunnam says he and McNelis are still working on their relationship.

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"You gotta work hard," he said. "I saw my girlfriend once or twice in 10 months... She is incredibly understanding and supportive in letting me go and do these things, but I repay her with total loyalty. She always knows I am going to come back at the end of it. Once there is that level of trust in a relationship, you can allow each other to grow and evolve and go and do weird and wonderful [things]."

Hunnam already has more "weird and wonderful" projects in the pipeline, however, as he revealed he's already signed on for more King Arthur.

"I signed on for three. I'd do as many as Guy wanted to do," he said.

King Arthur: Legend of the Sword hits theaters on May 12.

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