Exciting! Worked four years at Macca's. Probably taught me more than my undergraduate degree!
My #1 tip is, just be prepared to volunteer positive information. So many people don't want to come off arrogant but all an interviewer is trying to do is to determine whether or not you'd be a good employee. Sometimes all you need to do is say "I'd make a great employee" in a convincing enough tone and, well, shit - if you say so!
Think less about the questions that could be asked and more about the positive things you have to say about yourself and McDonald's.
(Also, to the young folk interviewing for fast food or retail - act like your job will be your priority. No one wants to hear than you just want to spend 3 hours a week at the place so you can put it on your resume. You need to demonstrate that you can provide value. As a 15-16 year old, you provide value by doing good work for shit all money, having good/flexibility availabilities and being enthusiastic about the opportunity).