Cassandra Clare has made a post on her tumblr regarding her involvement with the The Mortal Instruments movie:
I did want to add — I have of course been receiving a ton of messages about the movie: from people who loved it, people who hated it, people who are mad at me for changes made in the adaptation from book to movie, people who want me to give them Jamie Campbell Bower’s phone number immediately or they will be mad at me… It’s a little overwhelming, but I will just say:
I’ve always said I didn’t have ultimate control over this movie. (I said it on my website, and here, seven months ago and a year ago : “When you give over your movie rights you have to accept that decisions about how to adapt your books will be made without you. ) I clarified my position and influence re: the movie here a month ago: : ’The producers and director and production designer were all nice enough to come to me to consult about various things. I gave my opinion. Sometimes they took it. Sometimes they didn’t take it. Sometimes they took part of it but not the rest. Some things I was never asked about and was as surprised as everyone else to see.”
Interestingly, even being on set doesn’t change that - about 60% of what I saw filmed, including a ton of location shooting in ‘Idris’ and flashbacks, didn’t make it in — I hope it’s on the DVD because I think you guys would like it. None of that is a criticism of the movie — but often if you have questions about why things were changed, or when or how, I won’t know the answer, because I don’t know either. I just hope you do know that whenever I was asked for input, I gave it with what I thought you guys would want in the forefront of my mind.
In the end, a film is an interpretation: it’s the director’s vision of a book. If you don’t like it, you’re not a bad fan. If you do like it you’re not a bad fan. If you have a ton of strong opinions well, you’re a fan. :) It’s the nature of the beast. The books will always be the books and nothing changes that.
Source: Cassandra Clare's tumblr