Well, you have to have every character in the book in the movie (my opinion) because it would've been pointless for JKR to put a character in the series that doesn't even contribute to the books more than once. Well, maybe not. Even a character that only appears once is a big part in the film. If they don't cast Luna, then what about the DA meetings and what goes on there? When the kids are fighting Voldemort in the Ministry of Magic, Luna was there, right? I just think it'd be stupid to have her in the movie, introduce her in the wrong part of the movie, and not understand why she's there. The film makers have to take the perspective of someone who hasn't read the books yet. People watch the movie who haven't read the book. If they don't know how the book goes, they're already missing out on things in the movie... if you don't put certain characters in that make a difference in the movie, they'll be missing something. I dunno, take my mom for example. She watches the movies, but doesn't read the books. She doesn't understand certain events that don't make sense, and since I've read the books, I have to fill her in on what the film makers left out. I'm just saying, with out all the characters and events, it's just stupid to do so.