The 47-year-old made a drastic metamorphosis and lost so much weight for the role that his co-star, Emily Blunt, described him as “emaciated” after going to great measures to play J. Robert Oppenheimer, the “father of the atomic bomb.”

“He had such a massive undertaking. And he could only eat an almond every day,” Emily, who plays Cillian’s on-screen wife, biologist Kitty Oppenheimer, told Extra in a recent interview.

During a separate interview with Matt Damon, who plays Leslie Groves, the Manhattan Project’s director, the duo revealed that Cillian would avoid joining the cast for meals due to his rigorous diet.

“Every night, we invited Cillian to dinner, but he never showed up.” “He was losing so much weight for the role that he just didn’t eat dinner, ever,” Matt told Entertainment Tonight, with Emily adding that Cillian survived on “one almond most nights or a little slice of apple.”

Cillian has also spoken openly about the “overwhelming” lengths he went to represent Oppenheimer, whom he described as living on smokes and alcohol. “I love acting with my body, and Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right,” he explained to 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, and existed on martinis and cigarettes,” he explained.

Despite his unhealthy approach to character development, the Peaky Blinders actor admitted that the job of Oppenheimer was so demanding that he didn’t have time to consider his lack of nourishment. “You’re on a [expletive] train that’s just bombing. The sound is bang, bang, bang, bang. You sleep for a few hours, wake up, and bang it again,” he explained to The Guardian.

“I was running on wild energy; I crossed a level where I wasn’t concerned about food or anything…But it was appropriate since the character was like that. He was never hungry.”

Cillian does not recommend that others imitate his extreme weight loss tactics, and he has declined to reveal how much weight he shed for the role. “I don’t want it to be, ‘Cillian lost x amount of weight for the part,’” he told the site.

“You become a little competitive with yourself, which is unhealthy,” he continued. “I don’t recommend it.”

The film Oppenheimer, which opens in theaters on Friday, is based on Kai Bird and Martin J. Sherwin’s Pulitzer Prize-winning biography American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer, which was published in 2005.

It describes Oppenheimer’s life (1904-1967), his efforts to produce the atomic bomb during WWII, and his emotional agony as he faced “the moral consequences of scientific progress.”