Q. & A.: Cassandra Clare on 'The Mortal Instruments' "City of Bones," the film adaptation of the first book in Cassandra Clare's young adult fantasy series "The Mortal Instruments," opened last week. In a recent phone interview, Ms. Clare spoke about the film, her forthcoming middle-grade series, writing with Holly Black (author of "Doll Bones" and "The Spiderwick Chronicles") and what readers can expect from "City of Heavenly Fire," the final novel in the "Mortal Instruments" series, due next May. Below are edited excerpts from the conversation: Was there any scene or character you were particularly excited to see in the film? Magnus Bane, who is a trickster figure. I was worried they would cut him. I had to tell them he was a very important character who becomes more important. I was excited to see his scenes in action. Are there any specific things you had a say in during the adaption process? I had a lot of say in casting. They definitely invited me into the casting process and showed me audition tapes, and every person cast was a conversation about the pros and cons and how they fit in with the larger puzzle of the character. Would you ever consider a change in career? Or is there something you would really like to do? Well, one thing I've always been really excited to do was write for a slightly younger group and do something that was a little bit more of a coming-of-age story. I wanted to do something that would span a bigger chunk of time. So my friend, the wonderful writer of "The Spiderwick Chronicles," Holly Black, and I collaborated on a project. The first book comes out next year, and it's called "The Iron Trial." How do you co-write something? It's a very "togethery" process for us. I know people who do it differently. But for us we have to be in the same place and we have one computer and we write into a master document. We wanted to have a really seamless feel, like it was all written by one person. You can't tell the parts that she wrote or the parts that I wrote. Do you have a favorite character that you've either written yourself or read in another book? In my own work, I don't have favorite characters, but I have characters that I relate to the most. And I relate the most to Simon from "The Mortal Instruments," and also Tessa from "The Infernal Devices." They're more sort of bookish and shy characters. But in other people's books I tend to love the really daredevil-y characters. I love Finnick from "The Hunger Games." And I think, probably, my favorite character of all time is Sherlock Holmes. Could you describe your writing routine? I prefer to be out of my house because I find that otherwise I tend to get distracted by mundane details like my cat, things that need cleaning up, the laundry. And in terms of the thing I need to have with me, I would say music. I have specific playlists for different books and characters. So, I need to have those with me. It helps me get into the mindset of the book. Anything else you want to share with your fans about the film or the books? You're always worried, "Is the movie going to stay true to the book?" But they should remember it's an interpretation of the books, and I want every reader to feel that their interpretation is the true interpretation. The last book in the series is coming out next May. I hope that they are excited. I'm really looking forward to experiencing the end of this story with this readership that's grown over these years since we first started with the characters. I would say for them to look forward to some romance, some adventure and some maybe unexpected deaths. Blanca Melendez is a Times Scholar and intern at The New York Times Book Review.