Talk about a major transformation! Director Danny Boyle sat down with HuffPost Live on Wednesday, Oct. 7, and recounted how fully Michael Fassbender, the star of the upcoming movie Steve Jobs, transformed into the late Apple co-founder.

“I came onto the set and I saw this guy and I had this nightmare, because as a director, you want to try and know all the crew’s names,” Boyle, 58, recalled of working on an early scene where Steve Wozniak (played by Seth Rogen) and Jobs were toiling away in a garage, working on what would eventually become a personal computer. “I remember thinking, ‘Who’s that?! That’s someone I don’t know! Oh no … that’s Michael!’ So there was that moment where he changed somehow.”

The director, who won an Oscar for Slumdog Millionaire, explained that it wasn’t so much the actor’s looks that had altered and made him unrecognizable. “It’s this internal thing with great actors,” Boyle said. “You look at him in all these performances and he is Michael Fassbender, but on the other hand, he’s not! … He goes through a whole journey where he transforms internally. And sometimes, it’s manifested externally.”

Boyle said that by the time they were filming the final section of the film, everyone was “gobsmacked” by Fassbender’s change into Jobs. 

The biographical film, which also stars Kate Winslet and Jeff Daniels, opens on Oct. 23.

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