Opens Aug 23

By Sara Vilkomerson
April 12, 2013 at 04:00 AM EDT

Long before she was cast as feisty heroine Clary Fray in The Mortal Instruments, Lily Collins (Mirror Mirror) was a fan of Cassandra Clare’s best-selling young-adult fantasy series. ”I felt like I had gotten to know Clary, and I felt so close to her before I knew that I was going to be her,” says the actress.

But being Clary — a New York City teen who discovers her role in a supernatural world after her mother is abducted by a demon — isn’t always easy. ”The poor f—er has so many emotional things to go through in every scene,” says costar Jamie Campbell Bower (of Twilight fame), who plays the mysterious and otherworldly heartthrob Jace Wayland. To figure out his own complicated character — ”he’s like a ginormous monkey puzzle, isn’t he?” — the actor turned back to the books for guidance. ”If you didn’t read them, you’d be kind of stupid, I’d think,” he says with a laugh. ”You’ve effectively been given a bible. Life can be so much easier if it’s mapped out for you.”

With its healthy sprinkling of magic (good and bad), love triangles, and thrilling action, Mortal Instruments has built up a devoted following, which Sony is no doubt hoping will lead to Twilight-size box office. But there’s a downside to managing fan expectations, as Campbell Bower discovered when news of his casting met with online outrage. The actor admits his feelings were hurt. ”I had to sit back and assess why I was doing this [role],” he says, ”and it’s because I love it.”

Luckily, Campbell Bower had Clare’s backing. ”I know intellectually that when there’s a really strong reaction about someone being cast, it’s great, because it means people care,” says the author, noting that Robert Pattinson suffered a similar backlash when he was cast as Twilight‘s Edward Cullen. Clare, who cameos in the film as a partygoer (she’ll be wearing cat ears), is confident that the storm will blow over. ”I’ve seen Jamie play Jace and he’s perfect,” she says.

As for Collins, Mortal Instruments devotees are never far from her mind. ”I really hope I did Clary justice, because I’m a fan too,” says the actress. ”I have high expectations of my literary heroines.” Don’t we all?