Most important thing: relax. No, seriously. The interviews are extremely laid-back, and the interviewers (and everyone else) is very friendly. Everyone in my interview group (all of whom are my classmates) staggered out of the interview mumbling "They're so nice... they're so nice..." As far as I know, this hasn't changed.
Know the basics. Be able to give an interesting answer to "why do you want to be a doctor." Know something about osteopathic medicine. Make sure you have a question or two to ask. Other that that... be interesting. One guy in my class mentioned doing card tricks in his personal statement, and was asked to demonstrate. (I'm a little vague on the details- I think that he didn't bring the cards that day, but brought them to other interviews and actually did break them out. I could be wrong.)
One of the rumors going around right now is that the school is only interviewing for the wait-list. I have no idea if this is true or not... but I can say that the school takes many people off the wait-list. I know of at least one person who got the call during orientation week. Some people might have gotten called even later... I wouldn't be surprised. SOM only interviews people that they are very interested in, so you have a solid chance.
One of the other posters was correct. The interview is really laid back, one of the easiest that I had last interview season. I would say know why you want to go to UMDNJ. Great facilities, small class size, adequate facilities for the size of the student body, diversity, everyone seems friendly and welcoming. Good reputation and they do well in the boards. Now you should be confident but not cocky!!!Know that you are worthy of an acceptance. Smile and be interested in the interview. Lean towards the interviewer. Remember their name and make sure to shake hands and say "Thank you (interviewer) for this opportunity" as you do it. Make sure you are nice even in the reception area, not only to the interviewees, but to staff that may walk by. These may be the same committee members that will accept you. As a matter of fact act like you are interviewing once you enter the building. I was accepted last year and hopefully you will to. Goodluck and expect them to ask you why Osteopathic medicine and why UMDNJ?
Why do they bother interviewing for the wait-list? Honestly, because they have to. Because most schools send out acceptances throughout the year, applicants (the lucky ones, at least!) can be accepted at one school in September and at another in April... or in July. If School A accepts you first and you get good news from School B, your first choice, later on in the year, that frees up a spot at School A... which may be the first choice of someone accepted by School C, which in turn frees up another spot... and so forth. This happens through July, and any school that ignores this fact will have a much smaller class size than anticipated.
Interviewing for a wait-list isn't very exciting- and let me just repeat that I don't know if that's what's going on! That's something that I read on the boards, not heard at the school- but it's still worthwhile. A wait-list letter from SOM means that they like you, they like you a lot... but that they don't quite have the space- and that's something that changes throughout the year. Everyone hates wait-lists... but it's much better to be on one than to just be rejected out of hand.