'Mortal Instruments' Author Thought Movie Would 'Never Really Happen'

'It's like being at Disneyland where all of the rides are based on my book,' Cassandra Clare told MTV News.

From printed page to silver screen: It's been one wild ride for author Cassandra Clare and her "Mortal Instruments" series.

" 'Surreal' is the word," the author told MTV News' Josh Horowitz while on the Toronto-based set of the urban fantasy. "My husband says it's like being at Disneyland where all of the rides are based on my book. It has gone through a long process."

A long process, yes, but not an unexpected one for Clare, who has movie-making in her blood.

"My grandfather was a movie producer. I grew up in the film industry," she explained. "I always knew movies aren't really happening until they're actually filming, and I thought this will never really happen. And then when they got [director] Harald [Zwart] on board, I really started to believe it could because he has such a passionate commitment to the project, and he's a very emotion-focused director. And he really wanted to focus on the characters and their relationships to each other with the special effects second. And I thought that was a really great quality in a fantasy-film director."

Clare did admit that it took some time for it to all really sink in, recalling her aha moment:

"We were shooting in the subway, and they had closed off part of the subway for shooting, and I walked down the stairs to get to the set, and I saw Kevin Zegers in his full costume with runes and everything. And all the commuters were passing him by like they didn't see him. And I just thought, 'Oh my god. It's like we're in my book! He's here. He's invisible. It's like I've walked into a page of my book.' It was amazing."

And Clare wasn't just an observer on set. Like many authors before her (Stephenie Meyer in "Twilight" comes to mind), she'll make a cameo in the movie based on her best-selling series(hence her garb during our interview).

"I don't actually wear the cat ears all the time," she said. "I'm having a cameo at a very special party in the middle of the movie. I'm very excited about it."

For the uninitiated, the film follows Brooklyn teen Clary Fray, who after her mother's disappearance turns to an ancient race of demon destroyers known as Shadowhunters for help. Teaming with the impossibly beautifully and infinitely snarky Jace Wayland, Clary delves deeper into their mysterious world, learning she may be more connected to it than she ever could have imagined.

Actress Lily Collins plays the lead role of Clary, and as she told MTV News, she was pleased to have the author on hand while filming.

"She's so lovely," Collins enthused of Clare. "She's been here for a couple weeks just to take it all in. Just to have her support on this is so amazing. We're really lucky to have her on set. Just to see her smiling and be part of this process. Especially for her, this is huge. To have something you've created brought to the screen and have people so passionate behind it. It must be an absolute dream for her."

"The Mortal Instruments: City of Bones," co-starring Jamie Campbell Bower, Lena Headey and Jonathan Rhys Meyers, opens August 21.

Check out everything we've got on "The Mortal Instruments: City of Bones."