Oppenheimer's Cillian Murphy essentially existed on "an almond a day," according to costar Emily Blunt.
But he's hardly the first star to undergo a drastic transformation for a film.
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.