The 28-year-old actor played real-life Israeli adventurer Yossi Ghinsberg, who was left stranded in an uncharted part of the Bolivian Amazon jungle for three weeks in 1981. He foraged for fruit and eggs from nests to survive, losing 35lbs as hunger, infection and disease took its toll.
But while Radcliffe has denied following the same diet for two months in preparation, his meagre diet didn't consist of much more than that.
"I ate just one chicken breast and a protein bar for two weeks before the shoot in order to look thin," he said.
"It's not recommended, it's a really unsafe way to use weight. It was horrible but it was worth it for the meal afterwards."
"I'm not a method actor, but it would seem weird if I was playing this guy stuck in the jungle and going home, having a lovely steak dinner at the end of the day."