I was there last year. I would say the interview was very social and tons of parties, although I am not sure if every psychology interview is similar. As you can see from the schedule below (you probably already got one), two parties on Friday night. (there was a typo for Friday 2/26 ) And it is not going to be an 1-hour or 2-hour event, it will probably keep going past midnight. If you are very good with parties and absolutely love parties, you will be fine. But for me personally, I was really tired towards the end... While good news is, they will have amazing Hors d'Oeuvres and tons of alcohols.
Also, remember that you are evaluated outside faculty meetings. Although PI is the one who will make the final decision, he/she will ask current lab members for their opinions about the applicants. That being said, during the Happy Hour on Thursday night, try to find PhD students / post-docs currently working in your PI's lab and talk to them. Another time you can interact with current lab members is during your free times on the interview day. If you have some spare time, definitely go to the lab and meet people.
If you don't mind me asking, who is the primary PI that you will be interviewing with?