i just went to visit my new school for a third time and talked with ~10 faculty members for the sole purpose of trying to find a rotation/possibly scan the horizon for a PhD mentor. it's very, very laid back - you hear about their research, talk a little about why you're interested in what they're doing and just generally have an easygoing conversation. i wouldn't stress about it at all. second look was the same, except i only got to talk to 3 faculty members.
all of the professors i talked to actually DID have my application info and i was actually asked questions about my personal statement (which NEVER happened during the interviews at this particular school). however, the questions were of mere interest, certainly nothing to worry about. as itsallthesame said, most just commented on my recommenders (you'll find out just how small of a world it really is) and briefly on my previous research experience. just go and have fun, for reals.
more specifically, the method that i came up with was to go talk to these professors with only a very basic knowledge of what they did - i.e. apoptosis, modeling human disease in mice, targeted gene therapy, etc - and then have them try to convince you its a cool topic for research. then afterwards, pick your top 3 or 4 and read a whole bunch of papers on them. after that, it should be pretty easy to pick your favorite PI with whom to rotate this summer (or whenever). if you're worried about lab atmosphere, the best way to find out how a lab operates is to get in touch with the students. the PIs all willingly gave me names and emails of people i should talk to, and frankly they encouraged it as well.
