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