If you thought Christopher Nolan's Oppenheimer was nuts, just wait until you hear what went into making it.

After all, Cillian Murphy was looking to be explosive while transforming into J. Robert Oppenheimer, a.k.a. the father of the atomic bomb. 

"Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right," the 47-year-old told the New York Times of the scientist, who died from cancer in 1967. "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."

The Irish actor's diet was a bit different, but just as jarring, according to his costar Emily Blunt, who plays Oppenheimer's wife Kitty. 

"He had such a monumental undertaking," she told Extra. "And he could only eat, like, an almond every day."

And, reader, he doesn't recommend you try this at home.

"You become competitive with yourself a little bit which is not healthy," he told The Guardian, refusing to share just how many pounds he dropped. "I don't want it to be, ‘Cillian lost x weight for the part.'"

And if you have, you know, gone to the movie theaters (or streamed anything at home), you know he's hardly the first shape up for a role—and he won't be the last.

Keep reading to see stars like Tom Hanks and Anne Hathaway who have undergone drastic transformations on screen...

Plus, Oppenheimer (also starring Matt Damon and Robert Downey Jr.) is out now.

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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