The best thing you can do is relax and be yourself. This interview process isn't meant to stress you but to see if you are a fit into their model of serving the underserved. The Thursday night social is a great way to break the ice, meet some of the faculty and tour the facility. The ambassadors there are also a great resource for advice and information (they had no influence on your application results so you could be as candid, at least when I was a first year).
During the interview day itself there are a lot of lectures on what the school is about then the interview is about half an hour with two faculty members. During my interview we talked about books for most of it. The thing to keep in mind is that they are trying to serve the underserved, and so you should embody that somehow. Also knowing what a CHC is beforehand might come in handy.