Super relaxed & conversational. The admissions & faculty are super personable. If I knew how chill it was going to be, I wouldn't have been freaking out like I was. You should be good if you review the interview feedback section here on SDN. I was in the afternoon session, so our day started with lunch, then our 1-1 interview (I noticed guys interviewed with guy professors & same for the ladies), then an academic profile review session, & then a tour! The day lasted from 11:45 to 4:15ish.
One tip I personally should have paid more attention to - treat the entire day as an interview, not just that 30 minutes to half hour. The admissions committee members (who decide if you get in) will be talking to you on & off throughout the day, so be careful not to be on your phone, etc. while they're just casually talking to each of you in the waiting area. Not that you wouldn't know that already, but just something to be mindful of. Good luck!