I went on my first interview (it was all day) a couple of weeks ago. I don't have any info on your specific field but I will tell you a little about my interview anyway....
1) I wore a suit-everyone I spoke with advised me to "show respect and wear a suit." It was expensive and uncomfortable but it was just for one day so whateva. I recommend that you ask people in your field if you are unsure.
2) The program sent me a detailed schedule a few days prior to the interview. This was helpful because it allowed me to see exactly who I was meeting and review relevant info about each person. FYI- I had short interviews with every person in the department (roughly 15 people, including phd students) and an additional 5 people from other departments. NOTE: I asked my POI for a brief overview of what to expect and he gladly forwarded the schedule
3) They took me out to lunch and dinner. I cannot stress how important the non-academic setting is for evaluating you. Most schools that ask for interviews are looking at your "overall fit," i.e., can we deal with this person for several years?
Anyway, good luck and i hope my meanderings help.
BTW, I was told at the end of the day how much everyone liked me and I got an acceptance about 2 weeks later.