I would know your research really well. Several applicants who sat next to me during the interview came out of their faculty interview really shocked and scared because they didn't realize how much detail the faculty interviewer was going to throw at them. So know your research well. Also, don't make stuff up, if you don't know something, be honest and say that you're not sure, but you can definitely find out that information later on. The school is looking for honest, hard-working applicants, not BSers.
The essay is based on a topic that Dean Featherstone talks about during the interview day. If you pay close attention to what he says throughout the day and think about stuff to write about along the way, you will be set when they give you a quote to write about.
Finally, please smile =) I saw like a bunch of nervous applicants roaming thru the 4th floor sim lab halls and I had to tell them to smile because they looked so sad. It's your interview day, make the best of it, because an interview here means you have a shot at coming to our school. Hope that helps.