*Please keep in mind that I am a grad student, and have no experience actually making decisions regarding grad school admissions. I'm just giving my perspective from being a grad student in a biomedical sciences PhD program, seeing which students get in and which don't, and hearing feedback from professors*
This advice will mostly be about the general interview days, not necessarily the actual interviews with professors - their questions, in my experience, vary widely, and I can't begin to know what they are all looking for.
What I DO know, based on being friends with the student representative on the admissions committee (everything is sworn to secrecy about individual candidates, of course, but there isn't any rule against sharing information about the process in general) is this: once the committee has discussed the "meat" of the candidate's application (GPA, test scores, interview forms, etc), they also look at feedback from the graduate students. Most of the time, the grad student feedback probably won't matter, unless there was a MAJOR red flag, but for students that are border-line, grad student feedback can help set the order in which candidates will receive an offer.
So, onto the advice...
- Don't hit on current students (or anyone, for that matter). There is a time and a place for romantic behavior, and grad school interviews are not the time. Just don't.
- Don't dress inappropriately. At least make sure nothing is hanging out before your interviews...(true story: I gave a girl I interviewed with a safety pin because she forgot her camisole and her boobs were hanging out of her shirt. She's lucky I had a pin!) We also have students-only meals that are really casual, but we don't want to see too much of you, inappropriate slogans, or dirty clothes. If our first impression of you is a bad one...we're probably not going to end up giving you good feedback. Harsh, but true.
- Don't ask stupid questions. You should definitely ask questions! Lots of them! But... do a little research first. Don't ask what students do on their "summers off" if you're interviewing with a PhD program that doesn't have summers off. Spending an hour grilling people about the safety of the city, or only asking about "bars where you can pick up chicks" is not going to make a good impression. Safety is important, and wanting to know about the bar scene is totally fine...but don't get so caught up on one topic that all of the students just want to be rid of you so they can stop talking about it.
- Don't admit to "enhancing" your application (or, you know, just don't do it in the first place!). If you admit to lying on your application or getting someone else to write your research statement for you, we WILL tell on you. Period.
Those are the big things I can think of right now. You'd think they wouldn't need to be said...but I've seen enough these last few years of interviews that I realize some people don't seem to get it.
Anyway, if you're interviewing this year, good luck! Have fun! Find a program that is a great fit!
I'd love to hear your interviewing horror stories, grad school or otherwise :)