"Twilight" director Catherine Hardwicke warned Robert Pattinson not to pursue co-star and future girlfriend Kristen Stewart on the first few days of shooting the movie - because she was underage.

The filmmaker had already tapped Stewart to play emotionally-guarded Bella Swan in early 2007 and she had narrowed down the candidates for her vampire love interest Edward Cullen when Pattinson walked in for an audition.

Hardwicke tells Elle magazine, "Every two hours, I had another guy come over to my house. Rob and Kristen sat down at the table and did the biology scene. I felt the sparks. I could see the attraction. Kristen was very vocal - she knew she had the strongest connection with Rob. I said, 'Let me have a day to see if it translates to the screen.'"

Watching from afar, Hardwicke knew the two were destined to become a couple so she pulled 21-year-old Pattinson aside to inform the British actor of America's age of consent laws, which would make a romance between the pair illegal until Stewart, then 17, reached her next birthday.

Hardwicke adds, "I warned him, 'She's underage, don't even think about it! It's a law in our country.'"

But that didn't keep Pattinson away - he and Stewart struck up a relationship on set and they've been dating ever since.