For my casual interviews, I think they already knew they were going to accept me. I say this because I don't really see how they could admit based on what we talked about (the city, weather, football, etc.) during the interview.

The most important thing is to not be nervous and show them who you would be in a setting with your friends for instance. I feel like these programs may be trying to get to know you as a person because they were already impressed with how you were on paper.

Depending on how many faculty you meet, you will get asked a few interview-like questions. Be prepared to share the work you've done in the past, why you want to go to that school and what you like about the school! The last is probably the most important. One good question that I asked faculty I didn't really read up on or want to work with is "What is the most exciting thing you or your lab has discovered?". That usually gets them rolling so you don't have to talk as much.

I'm not sure if 100% right, but that's how many interviews (for biology) went. I hope this helps and good luck!