Hardwicke revealed this when she gave the podcast crew of her home where she held the Twilight auditions in 2007, showing them into her bedroom where the actors kissed for their screen test.
"Rob and Kristen did the audition right here on the bed. They did the kissing scene and he fell off and landed right there on this floor," she said.
"Rob and Kristen auditioned on my bed, the kissing scene, Rob was so into it he fell off the bed. I'm like, 'Dude, calm down.' And I'm in there filming with my little video camera, whatever.
"And at the end, Kristen was like, 'It has to be Rob,'" Hardwicke said. "I could tell they had a lot of chemistry, and I'm like, 'Oh my God.' I thought, Kristen was 17. I don't want to get in some illegal things.
"So I remember I told Rob, 'By the way, Kristen is 17. In our country, it's illegal to have a sexual ...' And he's like, 'Oh, OK, whatever,'" she said.
After the auditions, Pattinson's casting had to be approved by the author of Twilight, Stephenie Meyer.
Meyer originally wanted Henry Cavill to portray Edward Cullen to Stewart's Bella Swan.
But in 2007 Cavill was 24 and looked too old to play a 17-year-old vampire.
"My opinions on the movie matter to no one. I have no influence on what goes on with the movie at all. No one is going to ask who I think should star in Twilight," Meyer wrote on her website in 2007 about her casting choice.
"The only actor I've ever seen who I think could come close to pulling off Edward Cullen is Henry Cavill.
"The most disappointing thing for me is losing my perfect Edward. Henry Cavill is now 24. Let us have a moment of quiet in which to mourn."
Stewart and Pattinson soon went on to become one of Hollywood's most famous real-life couples, before splitting in 2012 after Stewart was caught having an affair with Snow White and the Huntsman director Rupert Sanders.
