What should I expect in a College Visit Interview?

I’m going on my first college visit (Washington University in St. Louis), and they had a thing where I could do an interview. I’m doing it, but I have no idea what it is.

Am I being interviewed or is this an opportunity for me to ask them questions?
Does anything in this interview affect my admissions? Is the impression that I make going to affect my chances of getting in (positively or negatively)?
What should I wear? Some people I asked said super casual was ok - but others said that I should dress up for any interview and go for a full pantsuit look. (female btw).
Is there anything else I should know?