The Twilight vampire told Merrick, Dools and Ricki-Lee that meeting the 20-year-old actress for the first time was "weird".
"It was weird. I mean, everything about the pairing with Kristen kind of worked out for the story," he said.
"I went in to the audition having absolutely no idea of what to do and because of the way she was playing it kind of shocked me into doing a performance that I didn't realise I could do.
"I guess she brings a kind of toughness and also a kind of mystery to Bella, which is not really there in the book.
"She's kind of in a lot of ways, a damsel in distress in the book, and Kristen's very...there's a kind of fierce fierceness behind her, which I didn't really expect the actress who played Bella would have."
The British actor said he is still surprised at the success of Twilight and that its success puts a certain pressure on him.
"In the beginning I had no idea. It seems to get bigger and bigger and bigger every week that is going past," he said.
"I feel pressure to live up to making a good movie, but I mean the expectations are so diverse.""