Cillian Murphy
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."
Joaquin Phoenix
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."
J.K. Simmons
The Spider-Man alum shocked fans when he debuted his bulging biceps, which were displayed in Justice League.
Jake Gyllenhaal
"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."
Michael B. Jordan
The Black Panther star gained nearly 30 pounds of muscle to transform into a rookie-turned-professional boxer in Creed.
Chris Messina
The Mindy Project alum had to gain 40 pounds for his role in Live by Night.
Renée Zellweger
The Bridget Jones's Diary actress gained weight to portray the lovable character in the franchise's first and second films.
Matthew McConaughey
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.
Rooney Mara
The actress looked lean and strong for her starring role in The Girl With the Dragon Tattoo.
Anne Hathaway
The Les Misérables star took home an Oscar for her role in the musical.
Christian Bale
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.
Jake Gyllenhaal (Again)
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.
Tom Hanks
"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.
50 Cent
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.
Robert De Niro
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.