Most schools say that the interview is is a very important part of the application process.
It is an opportunity for you to tell the admissions committee those things which just didn't come across clearly in your application. Try to look at an interview as your chance to let your best qualities show through. Though I was never asked any really difficult questions, I have heard of this happening to other applicants. Some interviewers try to see if you're the kind of person who will pretend you know something that you don't. The best thing to do if you don't know the answer to a question is to be honest and admit ignorance.