The British wildman visited Cara's school when she was seven and she has been obsessed with him ever since, so when he called and asked if she'd be interested in becoming his latest celebrity sidekick she jumped at the chance.
The model and actress felt sure she could stomach eating bugs and snakes - the usual dietary requirements on Grylls' shows - but when he suggested they dine on a dead rat, she wasn't impressed.
"The worst part was we had just traversed down a hill... and I saw this tail sticking out, and I was like, 'What's that? Is that a rat?' And he was like, 'Perfect, that's our dinner...'," she tells TV chat show host Jimmy Fallon.
"It didn't smell that bad but it wasn't fresh, obviously!"
Cara faced another challenge after lying to producers about her scootering skills.
"They usually ask like, 'Are you scared of heights? Can you ride a motorcycle...?' And every time I get asked this on a (movie) job... I always think, 'I can learn how to'."
"(This time) they were like, 'Can you ride a scooter?' I was like, 'Of course...' but I didn't know I was gonna have to, start myself, this old Italian scooter. That was the hardest part of the whole thing. It took me an hour to get the thing going."