In the film, Hunnam plays Henri Charrière — nicknamed "Papillon" — who was a real-life safecracker who was wrongly convicted of murder and sent to a penal colony on Devil's Island in French Guiana.
While there, he met a man named Louis Dega (played by Rami Malek), who was a master counterfeiter. Together, the two men forged a bond and worked to escape the prison.
Flanagan plays a character named Masked Breton, who appears to be a smuggler that Papillon and Dega hire to help them get free.
Hunnam and Flanagan spent many years together on Sons of Anarchy, but interestingly enough, that is not where there friendship began. Hunnam recently revealed to Collider that the two actually met years prior.
"I did a film, years ago, called Green Street Hooligans, with Elijah Wood, and I convinced the director (Lexi Alexander) to hire him," Hunnam recalled. "He didn't read the script, so I went to a bar, seven days in a row, where I knew he drank and had seen him once before, with the script. I went and had a few pints, every night, waiting for him to show up. I totally stalked him."
"I just have always really admired him and think he's such an interesting actor," he continued. "Once we actually met and became friends, it sealed our fate. He's like my brother."
He also explained that he was mostly the one responsible for Flanagan ending up in Papillon as well.
"The idea was brought up to me, and then I got it over the line. I just wanted to work with Tommy, time and time again," Hunnam confessed, later adding, "He's definitely one of the tribe members of my little creative team."
He's even so much a fan of Flanagan's that he wants to write and direct a film for his friend to star in.
"I've always said the film that I want to write, which I haven't written yet because it's been a long incubation period, but it's a project that I would love to do, there's just been no money, is a tiny little love story, set in the north of England and I've always dreamt that Tommy would star in, if I ever got my act together to direct that film," Hunnam confessed. "That's always been the dream."
Sons of Anarchy fans can catch Hunnam and Flanagan on-screen together again when Papillon opens in theaters ion Friday, Aug. 24.