Robert Pattinson and his co-star Kristen Stewart at the London premiere of "Twilight"Ian West/PA

Dominance and masochism are key elements in the relationship between Bella Swan and Edward Cullen according to Robert Pattinson's co-star Kristen Stewart.

"You have to question their [Bella and Edward's] motivations - to watch two people so unhealthily devoted to each other," she says.

"They are definitely not the most pragmatic characters."

"The weirdest f**king themes run through this story - like dominance and masochism," she tells Interview magazine.

But Kristen, 19, clammed up when it came to talking about her off-set relationship with Pattinson.

“I don’t talk to anybody about my personal life, and maybe that perpetuates [the attention], too,” she said, though lately she claims she doesn’t have much a personal life at all.

“The only way for me not to have somebody know where I went the night before is if I didn’t go out at all."

Like Pattinson, Stewart is also caught in the glare of the fans' headlights, although, unlike Pattinson, she has not so far been stalked by fans on set.

"‘Twilight’ is literally how every conversation I have these days begins - whether it’s someone I’m meeting for the first time or someone I just haven’t seen in a while.

“The first thing I want to say to them is, ‘It’s insane! And, as a person, I can’t do anything!’ But then I think to myself, God damn it, shut the f**k up.'"