I think over-preparing is not necessarily a good thing (especially if you answers begin to sound rehearsed). There is no way to prepare for all of the possible questions that you could be asked. Obviously you should know why you want to go into medicine, why this school, your strengths and weaknesses, and your application. At the end of the day, if you are a genuine, personable, humble and compassionate person, and as long as you give honest answers, interviewing is not too bad. As long as you are engaged, interested, smile and make eye-contact, that is about half the battle. Its fine if you stumble and if you don't know the answer to a question right away.
Edit: I would add that I still think anyone applying to medical school should prepare (I definitely prepared). Do a couple of mock interviews and talk to yourself out loud. Sometimes what you are thinking may not come across the way you would like if spoken aloud. Do research a couple of nights, or the night before your interview on the school and make sure what you bring to the table, matches that particular school's strengths, curriculum and mission.
All the best!