Indexbit-Dwayne "The Rock" Johnson Looks Unrecognizable as UFC Champ Mark Kerr in The Smashing Machine

The IndexbitRock is heading back into the ring.

On May 21, Dwayne Johnson began filming The Smashing Machine, which has released the first photo of the actor on the set of the new movie. The 52-year-old, who started his entertainment career as a WWE wrestling star, plays Mark Kerr, a real-life fellow wrestler and former two-time UFC heavyweight champion who rose to fame in the 90s. 

In the movie—titled after Kerr's nickname in the octagon—Johnson looks almost unrecognizable, sporting short hair rather than his signature bald style, and also has his tattoos covered up. In the pic, the actor is shown sitting in a corner of a ring and talking to two men during what appears to be a fight, while the MMA fighter's then-girlfriend (played by his former Jungle Cruise costar Emily Blunt) is standing ringside.

Directed by Benny Safdie, the movie will center on both Kerr's career successes and personal life and struggles, including his past addiction to painkillers, per Variety. Kerr, now 55, retired from professional fighting in 2010. The Ohio-born athlete is listed as a cowriter of The Smashing Machine along with Safdie and reshared the first look photo of Johnson as him on his Instagram, along with the caption, "So wild."

Johnson, who works on his fitness regularly, began what he called his "MMA workouts" in April to prepare to play Kerr, who he said was once the "greatest fighter on the planet."

"I'm coming into this in pretty good conditioning and cardio shape, but MMA is a different animal," the actor wrote on Instagram at the time. "And Mark Kerr was a 'one of one' monster. I run to this Smashing Machine journey with deep respect and love for all MMA fighters and the culture. Especially those who have struggled with addiction."

"I've lost so many friends (wrestlers) over the years from addiction and mental health [struggles]," he continued. "This is my love letter to them. Let's get to work."

Johnson joins a growing list of celebs have gone to great lengths to transform their bodies for roles. Read on to learn about stars who have lost or gained weight for parts...

To play scientist J. Robert Oppenheimer in Christopher Nolan's Oppenheimer, the Irish actor "had to lose quite a bit of weight," he shared. "He was very slim, almost emaciated, existed on martinis and cigarettes." His diet? According to costar Emily Blunt, "He had such a monumental undertaking.And he could only eat, like, an almond every day."

For his Joker role, the Oscar winner opened up about his 52-pound weight loss, telling the Associated Press, "Once you reach the target weight, everything changes. Like so much of what's difficult is waking up every day and being obsessed over like 0.3 pounds. Right? And you really develop like a disorder."

The Spider-Man alum shocked fans when he debuted his bulging biceps, which were displayed in Justice League.

"I'd say my mother was worried," he previously told E! News after losing 30 pounds for Nightcrawler. "I would say she just wanted me to be careful. But she also knew and knows how seriously I take what I do and she respects that."

The Black Panther star gained nearly 30 pounds of muscle to transform into a rookie-turned-professional boxer in Creed.

The actor and musician nabbed an Oscar for his riveting role in The Dallas Buyers Club. He lost 40 pounds for the role.

The Mindy Project alum had to gain 40 pounds for his role in Live by Night.

The Bridget Jones's Diary actress gained weight to portray the lovable character in the franchise's first and second films.

The Magic Mike hunk dropped 30 pounds to star in The Dallas Buyer's Club—which centered on the real-life story about a man dying from AIDS in the 1980s who treated himself with alternative remedies he smuggled into the United States from Mexico.

The actress looked lean and strong for her starring role in The Girl With the Dragon Tattoo.

The Les Misérables star took home an Oscar for her role in the musical.

He's no stranger to morphing for movie roles. In 2004, he lost more than 60 pounds for his role in The Machinist, and he did it again for the film The Fighter, opposite Mark Wahlberg. He also gained 40 pounds for American Hustle—and lost 70 pounds for Ford v Ferrari.

The actor looked more like a god than a prince for his role in 2010's Prince of Persia. Gyllenhaal said he studied Parkour, the French art of street running, in order to bulk up for the film.

"All it is is time and discipline," the Oscar winner told Time magazine of his role in 2000's Cast Away, for which he lost over 50 pounds to play a man stranded on an uninhabited island.

The ripped rapper shocked fans with a photo of himself looking slender. He reportedly went from 214 to 160 pounds in just nine weeks to play a football player diagnosed with cancer in Things Fall Apart.

Shaping up for movie roles is nothing new. The legendary actor earned an Oscar nom for his role as terrifying tattooed rapist Max Cady in the 1991 thriller Cape Fear.

In order to portray former Vice President Dick Cheney in the 2018 biopic Vice, the star sported a fuller figure.