Cillian Murphy only ate 'an almond a DAY' to transform into J. Robert Oppenheimer who existed on 'cigarettes and martinis' - claims co-star Emily Blunt: 'He was so emaciated'

Cillian Murphy only ate an 'almond a day' to transform into the father of the atomic bomb, J. Robert Oppenheimer for new blockbuster, Oppenheimer.

In an interview with Extra, the star's on-screen wife Emily Blunt, 40, said the Peaky Blinders actor became 'so emaciated' as he invested himself in the physical transformation for the film.

She said: 'He had such a monumental undertaking. And he could only eat, like, an almond every day. He was so emaciated.'

And Murphy, 47, all but admitted Blunt was right - the actor  told The New York Times in May: '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.'

But Murphy doesn't want the headlines to be about the diet he undertook for the role. 

'I don't want it to be, "Cillian lost X weight for the part,"' he admitted to The Guardian earlier this month.

And he made it clear that his extreme diet wasn't safe and should not be tried by other people. 

'You become competitive with yourself a little bit, which is not healthy,' he said, adding, 'I don't advise it.'

The Inception actor also explained that he didn't want reveal what foods he cut out to play the pioneering scientist or how much weight he lost.

And the breakneck pace of filming helped Murphy reach his weight loss goal.

'It’s like you’re on this f***ing train that’s just bombing. It’s bang, bang, bang, bang. You sleep for a few hours, get up, bang it again,' he told The Guardian.

'I was running on crazy energy; I went over a threshold to where I was not worrying about food or anything. I was so in it, a state of hyper...something,' he explained.  

'But it was good because the character was like that. He never ate.”

Oppenheimer, which hits studios on July 21 also stars Matt Damon, 52, Florence Pugh, 27, Robert Downey, Jr., 58, and Rami Malek, 42.

The outlet asked Murphy if he's happy with his performance in Oppenheimer.

'I am, yeah,' he said. 'I don't like watching myself – it's like, "Oh, f***ing hell"– but it's an extraordinary piece of work.'

'Very provocative and powerful. It feels sometimes like a biopic, sometimes like a thriller, sometimes like a horror,' he said. 

'It's going to knock people out,' he adds. 'What [Nolan] does with film, it f***s you up a little bit.'