I just got back from my first interview and my advice is to learn as much as you can about your PI's work. Get a feel of their publication history by reading abstracts going back in their career and read a few of their papers (3-5) related to their current work. This is not a research project and no one will grill you on their current research - in fact, I found that my PI and the lab members were very excited to tell me about their work and having questions about the work that they were doing was not frowned upon. Be honest when you are not familiar with a topic they are speaking about - most are totally happy to explain it to you or point you to resources to learn more - but acting like you know will make you look phony.
Basically, know what you are interested in and want to talk about beforehand through your research, but be ready to learn a lot. Some of the research you will learn about is likely unpublished or starting up in the future and you won't know about it until you get there. Soak it all in, try to imagine how you could see yourself participating, and get excited about discussing it with top-notch researchers - they will definitely notice your enthusiasm.
At the end of the day, the faculty only bring in people for interview that they believe can do the work. The interview is to evaluate how well you will fit in with everyone there. So, don't worry about knowing everything heading in - this will come with time - and just try to learn as much as you can and be enthusiastic about the possibility of working in lab that is doing key research in your topic of interest!