Here's the tiny bit that I know. I've been accepted to one school and I'm still going to the weekend in late March. Since I'm already in and know that they really want me, they'll probably be trying hard to convince me to stick with them. On the other hand, if you haven't been explicitly offered admission, then odds are you will be "competing" against other prospectives. That said, don't get too worked up- just be yourself, re-read your application materials so you are ready for their questions and have fun (they are paying for your trip, after all)!
Interviews can vary greatly from softball, get-to-know-you questions like "So what's your favorite thing about field X?" to hard-core field-related questions like "How do you read Malory's Morte d'Arthur in light of the War of the Roses?" In my opinion, you can't be frank enough with your POI. Politely ask how many spots are still available and how many are being interviewed. This should let you know how serious they are, or how certain they feel. Bottom line: they're using their skimpy funding to fly you across the country- they want you! So relax and shine while you're there. You can only do your best.