How this 'Game of Thrones' star landed the role of a lifetime via a cold-message on Facebook

As the current World's Strongest Man title-holder, Hafthor Bjornsson can pull a 20-ton tractor trailer all by himself. But, when it comes to landing the job of a lifetime as The Mountain on HBO's "Game of Thrones," it took an unsolicited Facebook Messenger message to get the job done. 

"When I got it, I read it [and] I didn't even respond, because, you know, I just thought it was someone joking around," Bjornsson tells CNBC Make It. "I just I didn't take it seriously."

Bjornsson, 30, is a native of Iceland, where he started his athletic career as a teenager, playing professional basketball for three different Icelandic professional teams and competing in strongman events for nearly a decade.

The six-foot-nine, 400-pound Bjornsson is a big name on the strongman circuit — he won the title of World's Strongest Man for the first time in 2018 after finishing either second or third in the annual contest in each of the previous six years. But, most of his fans recognize him for his acting.

Since 2014, Bjornsson has portrayed Ser Gregor "The Mountain" Clegane on massively popular fantasy series "Game of Thrones." Bjornsson never set out to become an actor, but Hollywood came calling anyway in 2013, when he received an unsolicited Facebook message from a member of the "Game of Thrones" casting department.

The producers of "Game of Thrones," which was filming some of its scenes in Bjornsson's native Iceland, needed someone to portray the character of The Mountain, a brutish and enormous knight. (The role had already been filled by two different actors who each made a brief appearance in the show's first and second seasons, respectively. But, the "Game of Thrones" casting department was looking for someone new to tackle the role permanently.)

Bjornsson, who was just 24 at the time, was a fan of "Game of Thrones," but since he had no acting experience, he assumed the message he got from someone claiming to work on the show must have been a hoax.

Fortunately for him, the show's casting department was persistent. "A few days later, I got a phone call," Bjornsson says. "And, yeah, I guess it was serious and they really wanted me in the show."

Clearly, Bjornsson's massive size and impressive athleticism had caught the attention of someone in the "Game of Thrones" camp. And once Bjornsson finally realized that the overtures were legitimate, he spoke to the show's casting director and then met with C.C. Smiff, the award-winning stunt coordinator who oversees all of the many sword-fighting sequences on "Game of Thrones."

Bjornsson auditioned for the show by reading a few lines, but he also showed Smiff that he was not just big, but also athletic enough to realistically handle a sword in a choreographed fight scene. "A lot of big guys can't bend down, have knee pain, are slow," Björnsson told Men's Health in 2017. "I'm fast. I'm flexible, especially for my size."

Smiff was indeed impressed. Bjornsson landed the job a few days later and started shooting for the fourth season of "Game of Thrones" in 2013. When the season aired, starting in April 2014, Bjornsson suddenly became famous for his acting, especially after a particularly gruesome fight scene in which his character gouges out the eyes of a rival called "the Red Viper."

That fight scene "definitely changed my life," Bjornsson tells CNBC Make It of the episode that was viewed by more than 7.1 million people when it premiered. "It go so much attention around the world. I didn't really realize how big it would turn out to be."

The graphic scene (which included a surprising plot twist) was so well received that it spurred numerous blog posts and online headlines around the world, while also helping to grow Bjornsson's social media following.

"I remember the day that was shown on television, my phone didn't stop because I had all the notifications on," Bjornsson recalls. "I got new followers each second."

Bjornsson now has over 1.6 million followers on Instagram and his newfound stardom (both on TV and as an athlete) helped him land new opportunities, including starring in a recent commercial for Krazy Glue. He also says it's sometimes hard to tell if he gets recognized more for being the World's Strongest Man or for his role on "Game of Thrones," which will air its eighth and final season in April.

"If I'm walking on the street, people recognize me more from 'Game of Thrones' than Strongman," he says. "It's close, but it's probably close to like 60-40 [ratio]."

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