The actress, who plays a warrior named Quorra in the upcoming movie, endured weeks of torturous workouts to whip her physique into top shape.
And she says she "knew we did something right" because towards the end of filming, her transformation was so dramatic even security on set demanded to see photo identification.
She explains, "I'd never done that before. I'd never really transformed. I got into shape... I did some boxing...
"Nobody realised it was me. After two months of shooting I walked off set to visit some of the other actors and I wasn't in my wig and wardrobe and the security guard stopped me and was like, 'Let me see your badge.' And I was like, 'Dude, I've been working here for two months! You know me!'