CC2014,
I've been in two so far, but my experience and from other stories I've heard, the "interview" (at least for geoscience) is more like a meet and greet. You are typically put up with a graduate student host in your department, and that person will tell you anything you need to know about the program. For both of mine, there is a day that is more generalized, and then a day that is specific to your specialty. On the "general" day, you will get group tours of labs/ collections/ offices/ campus, and given information about funding/ requirements/ etc. For the "specialty" day, you will have one on one meetings with your POI and relevant faculty members (these are very relaxed, and quite fun! I was just asked about my broad interests and philosophy of science). These days are so much fun, but crazy exhausting! There is hardly time to make a call or use the facilities! All food is taken care of, and typically there is an opportunity to talk to students and faculty over beer.
If you a preparing for one, I wouldn't worry about "studying up" too much (I did this, and it proved useless), but make sure you understand your reasons for wanting to enter their program, and be able to talk (generally) about your research experience and what you can bring to the program. Also, if you come in with the right questions, you can get a lot of really helpful information from the graduate students. Good Luck!