Hey mvanz, congrats on getting an interview!
I just had an interview, and early admit, for a direct entry program. The interview procress that I went though was a bit unique, we were in a group, but I got a good sense of what they are looking for.
Of course it is a good idea to think about questions and answers pertaining to your chosen specialty, why it is a good match for you, where you see yourself in the near and far future, ect. But, I don't think you should worry about having the perfect answers.
There is only so much you can prepare yourself because they are mostly just looking at what kind of person you are. They already know a lot about you by your transcripts, resume, and references. Now is their chance to get a look at you, your personality, and your social skills.
The best thing you can do is go in there with a positive and open-minded attitude. Dress nicely and conservatively, have good posture, smile, make eye-contact, shake hands, speak clearly and slowly. They want students that have more than a stellar application, they want people that are socially competant--not too arrogrant and not too nervous or withdrawn.
If they ask you a silly question, give them a silly answer. If they ask you a difficult question, take a moment to think about it. It is OK if you don't know an answer, more importantly show them how much you want to go to their school. Don't be afraid to show your true colors, and just be yourself. Remember, they are only human, and we can all spot someone that is acting unnaturally.
I have a friend that is an HR manager and she said that the most unfavorable quality in an applicant are those that answer the moment she is done asking a question and then just spew forth some "perfect-sounding" contrived answer.
So rest assured, if you are comfortable in your own skin and can put your nerves aside, you will do great!!! Good luck!