'Mortal Instruments': How Author Cassandra Clare Helped Bring the Book to Life (Video)

The writer found herself fielding late night phone calls from director Harald Zwart and was "really involved" in the casting process.

Mortal Instruments: City of Bones book fans needn't worry about the forthcoming film adaptation -- author Cassandra Clare has given it her stamp of approval.

"They were very good about asking for my input," Clare tells The Hollywood Reporter of the filmmakers, including director Harald Zwart, revealing that she was also "really involved" in casting the project.

Lily Collins stars as the story's protagonist, Clary Fray, who, through the disappearance of her mother (Lena Heady, Game of Thrones), comes to discover that she holds supernatural traits. Together with a fellow Shadowhunter named Jace (Jamie Campbell Bower; Twilight, Harry Potter), Clary must enter into a new world of demons and hunters to find her mom and save herself.

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"Jamie really astonished us all by how much he captured what Jace is like on the page," says Clare, adding that newcomer Godfrey Gao -- playing Brooklyn's High Warlock Magnus Bane -- most closely resembles her own mental picture of the character.

While Clare (nee Judith Rumelt) isn't credited on the film beyond the obligatory "based on the novel by" reference, she's hardly new to Hollywood. Her maternal grandfather, Max Rosenberg, was a veteran film producer whose credits include The Curse of Frankenstein and Tales From the Crypt, while Clare got her professional start in the 1990s working in The Hollywood Reporter's special issues department as an editorial assistant.

"It was my first ever job out of college," she laughs. "Very fond memories of working there. It was fantastic."

Clare says that her grandfather "was a huge believer in the power of storytelling" and gave her "fantastic books" as a child.

"He was a big enthusiast about mythology and folklore," she says, "and I think a lot of them have worked their way into the mythology for the City of Bones books."

And Zwart (The Karate Kid) says he kept Clare on "speed dial" during production.

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"I loved having her around because she's the one who thought of it all, obviously," Zwart tells THR. "I remember on one of the last takes we did, it was like, 'Oh, the line doesn't feel right' and both Lily and Jamie were like, 'There's something …'"

Cue Clare, who woke up to answer a late night phone call from the director and saved the scene.

For the author, who has penned five books in the Mortal Instruments series, with a sixth scheduled for release next March, it was her diehard fans' approval of the film that allowed her to breathe a sigh of relief.

"They just had the first screening for fans, and so I ran into some of my fans who have been fans for a really long time," she recalls. "They were like, 'We love it!' And when they told me that the loved it, that was the moment where I thought, 'OK.' The fact that they love it is the thing that meant the most to me."

Sony's first picture in a planned franchise hits theaters on Wednesday, Aug. 21. A sequel, titled City of Ashes and with Sigourney Weaver on tap to join the cast, is already in production.

"[I'm] very excited about the script," says Zwart. "It's gonna be even more complex and scary than the first one."

For more on City of Bones, including testimony from Mad Men's Jared Harris -- making his YA debut in the film, watch the video above.

Email: Sophie.Schillaci@THR.com

Twitter: @SophieSchillaci