Well this week, costars Emily Blunt and Matt Damon sat down with Extra, where they discussed Cillian's dedication to the role.
"Cillian had such a monumental undertaking," Emily shared. "And he could only eat, like, an almond every day."
"He was so emaciated," Emily added.
"The sheer volume of work that he had, and the level of detail that Christopher [Nolan] demands," Matt said. "There was always something for him to be thinking about. There was something coming down the pike."
Oppenheimer's diet, Cillian recently told IndieWire, consisted of "cigarettes and pipes — he would alternate between the two."
“I don’t want it to be, ‘Cillian lost x weight for the part.' I love acting with my body, and Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right. 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."
Wow. You can watch Emily and Matt's Extra interview here, and read Cillian's IndieWire interview here.