They ask questions about your application, your research, your volunteering. I don't think my interviewer asked "why DO". But they did ask me about one non-academic achievement. (In my case I talked about my hair donation record. Now at 14 inches! ^.^ My interviewer happened to be bald, so he was like, "I uh can't help out with that.") So definitely don't stress about it.
Be yourself and be genuine. Remember that you guys are more than just book smart and a bunch of numbers. Sure you need that...but you're also colorful creatures that bring life to the campus!
I think the only people they don't accept are jerks and liars. So as long as you aren't any of those, you'll do just fine! ^.^