The actors, who play onscreen lovers in the vampire movie franchise, have repeatedly denied they are more than just friends, but rumours heated up earlier this week (begs09Nov09) when they were pictured holding hands in Paris, France.
Hardwicke, who directed the first film in the franchise, initially warned Pattinson against dating the younger actress when she first cast them to star opposite each other.
But she reveals Stewart later decided she had to "give this a go and really try to be with" Pattinson.
Hardwicke tells Time magazine, "After I cast him, I told Rob, 'Don't even think about having a romance with her. She's under 18. You will be arrested.'... I didn't have a camera in the hotel room. I cannot say. But in terms of what Kristen told me directly, it didn't happen on the first movie. Nothing crossed the line while on the first film.
"I think it took a long time for Kristen to realise, OK, I've got to give this a go and really try to be with this person."
Stewart split from her longterm boyfriend, actor Michael Angarano, earlier this year (09).