Cillian Murphy's role in Oppenheimer pushed him to extreme lengths.

In fact, his co-star Emily Blunt—who portrayed Murphy's onscreen wife Kitty Oppenheimer—revealed that the Inception actor underwent a drastic transformation to play the role of J. Robert Oppenheimer, the father of the atomic bomb.

"He had such a monumental undertaking," she told Extra in an interview published July 7. "And he could only eat, like, an almond every day. He was so emaciated."

Murphy previously admitted that he was committed to a strict diet for the role in order to accurately capture the scientist's appearance.

"I love acting with my body, and Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right," Murphy told the New York Times in May. "I had to lose quite a bit of weight, and we worked with the costume and tailoring; he was very slim, almost emaciated, existed on martinis and cigarettes."

However, he warned that the undertaking was not something he would recommend to others.

"You become competitive with yourself a little bit, which is not healthy," the Peaky Blinders actor told The Guardian in July. "I don't advise it."

As for how much weight he lost? Murphy opted not to share a number, explaining, "Ach, no. I don't want it to be, 'Cillian lost x weight for the part.'"

In addition to his physical transformation, the 47-year-old said the fast-paced filming process allowed him to embrace the character a lot more.

"It's like you're on this f--king train that's just bombing," he told the outlet. "It's bang, bang, bang, bang. You sleep for a few hours, get up, bang it again. I was running on crazy energy; I went over a threshold to where I was not worrying about food or anything…But it was good because the character was like that. He never ate."

Oppenheimer hits theaters July 21. For more drastic movie transformations, keep reading...

Wahlberg shared a side-by-side photo of his previous toned physique alongside his look, which he achieved by gaining 20 pounds for his film, Stu. "From this 3 weeks ago, to this, now," he wrote, shouting out his chef, Lawrence Duran. "Thanks to @chef_lawrence_d cooking."

 

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 Parks & Recreation alum transformed into a fit superhero for Guardians of the Galaxy and never looked back.

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 actor bulked up for his roles in Whiplash and the Divergent franchise.

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 actor lost 25 pounds for his role as a heroin addict in Other People's Children.

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 teen heartthrob went from cute kid to beefy babe as werewolf Jacob Black in the Twilight sequel. "The day I finished Twilight, I came home and started bulking up," he told Interview magazine. "For New Moon, I'm 30 pounds heavier than I was in Twilight."

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. 

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