Best advise I can give. Don't worry about it and be yourself. I interviewed at 6 places last year (Case, Penn, Maryland, Tufts, USC, UCLA). I found that at 5 of the 6 the interviews were very chill. The best way to prepare is to do a little basic research about the school and have some good genuine reasons why you applied to that school. Also know what you put on your applications (both AADSAS and the secondary if applicable). You will look like a tool if you say something that contradicts you personal statement. Most of your interviewers will know your application better that you do at this point. Rather than try and guess what questions they will ask and have a great answer to them before hand think about what questions you have for them. They are going to ask you if you have questions for them. Have a few good ones in mind because that really helps you look like you care. Mostly just be yourself and relax. Most of my interviews were just trying to get to know me and make sure I wasn't a weirdo I felt.
Disclaimer: USC's interview day is pretty unique. Prepare for that one and actually read all the stuff they send you in the mail before your interview. Also don't be a tool and try and be the only one that speaks during the PBL. If you are going to speak try and make it intelligent. I think they value quality over quality although you need to speak enough to stand out.