Twilight co-stars Robert Pattinson and Kristen Stewart have undeniable chemistry both on and off screen, according to the film's director.

Rob and Kristen - who play star-crossed lovers Edward Cullen and Kristen Stewart in the vampire flick - are rumoured to have mixed business with pleasure during filming, despite the fact Kristen has a long term boyfriend.

Catherine Hardwicke tells a U.S. publication of the audition process the pair went through to make sure they were suitably matched.

"[During casting] I did the whole scene in my bedroom to be sure that these two people had sparks," Catherine told Life & Style.

"Are they intense? Are we going to believe it and love it?"

Rob and Kristen were informed they had to lock lips to ensure there was an earth shattering connection between the pair.

"I said, 'Yes, we really want you to kiss. I want to see if this works,'" she reveals.

"Because at the end of the day, if it just doesn't work, then it's not going to work for the whole movie. So I made them go ahead and kiss, and the sparks did fly!"

And the rest is history!