Why do you want to work here? Don't just say "I need a job". Say something about being interested in retailing, and you like movies a lot. Try to keep slipping that into your answers.
Have you ever dealt with/How would you deal with a difficult customer? Standard question. They're not interested in the answer itself, but how you answer it. Basically, in dealing with the public, you are going to encounter difficult people. You need to convey that you can deal with people, no matter how angry they are, and let it go. Tell them of a time when you dealt with a difficult person, and how you dealt with it. It's OK to tell them that you might ask for a short break after dealing with a really difficult person so that you could relax and refocus. It shows forethought, and that you're just like everyone else.
Do you mind working late/overtime? The answer to this is yes if you want the job. As the newbie, you're going to get the crappy hours and the worst shifts. But if you hang in there longer, especially if you are on good terms with whoever assignes the shifts, it gets better. But don't tell them that. Just say yes, you'll do whatever needs to be done. But if you have school, mention that too. They can't work you too late on a school night if you are still in HS.