What was your inspiration for writing the Mortal Instruments?
I wanted to write something that would combine elements of traditional high fantasy - an epic battle between good and evil, terrible monsters, brave heroes, enchanted swords - and recast it through a modern, urban lens. So you have the Shadowhunters, who are these very classic warriors following their millennia-old traditions, but in these urban, modern spaces: skyscrapers, warehouses, abandoned hotels, rock concerts. In fairy tales, it was the dark and mysterious forest outside the town that held the magic and danger. I wanted to create a world where the city has become the forest -  where these urban spaces hold their own enchantments, danger, mysteries and strange beauty. 

What was the most exciting part of filming? Was there a specific part of the book you couldn’t wait to see translated on screen?
Just seeing the world that has existed in my mind for so long come to life was amazing. The sets, the costumes, the actors... it was totally surreal to see my imaginary world manifest itself in front of me. I didn’t have one particular scene I couldn’t wait for, but I was really excited to see how the Institute came together.

How did you create the Mortal Instruments characters- who inspired you to write/create them?
That’s a tough question. You sit down and write, and these people show up. Sometimes they do what you expect, and other times they don’t! Some characters, like Jace and Isabelle, show up with a lot of attitude, very thoroughly themselves. Clary was inspired by my artist friends because, not being a visual artist myself, I’m fascinated by the way they see the world. I also deliberately gave Clary a lot of traits I associate with boy heroes in fiction: impulsiveness, recklessness, lack of regard for her own safety, the responsibility to protect her mother, best friend, boyfriend; the responsibility to save the world. I wanted to see how they would sit with a girl protagonist.
Then there was Magnus, who was meant to be a minor character but very quickly made it clear that he had other plans!

Do you have a favourite character, besides for Clary, of course?
I’m too close to them all to pick favorites, I even love the bad guys! But Simon is the most similar to me, while Magnus and Jace are the most fun to write. Clary I adore and admire.

How involved were you in the casting for the movie?
I got to see some of the audition tapes, and the casting director talked to me about possibilities - we spent a lot of hours going over and over footage and demo reels. I do think you can see my obsession with BBC dramas reflected in the final cast!

The Clary/Jace storyline in the books is quite controversial – were you worried about how this would translate on screen?
Is it controversial? I think the end of the first book is a shocker but certainly by the third or so there's nothing particularly controversial there. Which I think is not unusual in a love story: they usually have obstacles and sometimes the obstacles can skate the edge of taboo because the love needs to be forbidden - there's a teacher/student romance in Divergent and Vampire Academy, Katniss is supposed to kill Peeta, Edward wants to eat Bella - I think by now audiences are pretty used to the idea of an insuperable obstacle to a love story.

Which would you rather be, a Shadow Hunter or an Downworlder?
A Downworlder. They can kind of lay low and have somewhat normal lives, in certain cases. I’d last about half a second as a demon-killing soldier! I'd probably want to be a warlock, because then you can live forever without drinking blood. I don't think I'd like blood.

What can fans of the books expect from the movie?
I haven’t seen the finished product, but they can expect to see a movie that takes the fictional world we love and makes it real, and gorgeous! The film won’t be exactly the same as the book, because film works very differently as a medium than the written word does, but I think it will be a lot of fun. I also think the actors are perfect for their roles, and that they will really win readers' hearts.

It’s been revealed that Jessie J’s on the movie soundtrack. She’s a perfect fit! Did you have any part in getting  her involved?
I didn’t, nor in getting Demi Lovato or AFI or any of the other great artists. Universal Republic Records, who did the soundtrack for the Hunger Games, did it all.