EnchantedR

I completely agree with all these. 
Be polite to everyone. Everyone is evaluating you in some way, but also being a nice person is generally a good idea (whether or not you're at an interview). Don't be that guy who was noticeably rude to a waiter.
You're going to be living and working in these places, so you should definitely ask about what it's like to live in the area/specific questions about the area. It's great to ask professors during interviews about the city, but you should also ask about what their lab is like. Questions like "Do your students have fellowships?" "Do they go to conferences?" "Where is your funding from?" "What do your students go on to do?" "How many people are in your lab?" show that you know that there's more to being in a lab than just the research. You can love the research but the lab could not offer good opportunities, or not have funding, or is too big/small for your preferences, etc. Of course you can ask grad students the same questions.
Definitely take notes. If you have more than one interview, you're going to forget what you did or didn't like at every school by the time you're deciding. I took notes after each faculty meeting about what we talked about and what my impressions were. But taking notes during would be rude (these interviews are supposed to be like a conversation), so take them when there's nothing else going on. Feel free to take notes during powerpoint presentations.
I agree to this only to the extent that you should be aware roughly of what they do, but faculty profiles are often out of date. They won't expect you to know everything about their work, or to have read any papers. Only read papers if you're actually really interested. They will likely tell you about their work during the interview (if not, you should ask them about it). Don't refer to any notes during the interviews, but feel free to glance at the one sentence "Dr. X works on XYZ" you should have before you go into the interview.

I spoke with a professor yesterday who is new to my school but conducted PhD interviews at his previous university. He said getting an interview is the most difficult part of the process. Mainly during the interview process these programs are checking to make sure you are not a psychopath (haha!) and half as interesting as your application makes you seem. He stressed that most students forget their manners during these interviews. Always be polite, don't slouch in your seat, be dressed appropriately, when speaking with a faculty member be actively listening and do not interrupt them while speaking, etc. Most importantly, don't be on your phone and make sure it is silenced!!
He also said DO NOT speak about yourself unless you are directly asked a question and if you are asked, keep it brief. They have poured over your application and know it well, so don't drone on and on about yourself. He said be sure to ask questions to grad students "What are you studying? What do you do in lab? How are the classes here?" etc etc. Don't be afraid to ask about the city, how its like to live there and what are some good areas for housing? You can also ask the grad students about fellowships and funding. Ask professors if they are taking students or will be doing lab rotations, ask them about new developments in they research. He also said it is extremely important to have background knowledge on the professors you will be speaking with, read their faculty research profile and if you are extremely interested in them read a few of their papers. Make notes about their research so you can refer to it during the interview process. Be aware if a professor publishes very little or publishes consistently, how many students they have, and what kind of grants they have currently (or if they lack grants). He said that you can expect to encounter two types of faculty during interview weekends, those who want to talk about their research and recruit you to their lab or PI's who will grill you about your research background. Take note of your interactions with the PI, if your personalities clash then you might not want to request them for a lab rotation if you pick that program.
He also encouraged me to take notes about the program and my experience during break times throughout the day, obviously you're not going to remember every single detail of the 2+ days you'll be interviewing. Never take notes during interviews or during meals, only during designated break times. Be aware the faculty members are looking at you as a potential mentoree while grad students are looking for potential lab mates, everyone is watching you at all times! Good luck during your interviews

My friend in Harvard just got an interview on 12/31. This program only takes very few students, I'm afraid they sent out interviews all at once. Good luck in other programs!

I applied for the integrated program and they switch me to the genetics and development track. I got an email on 12/21.