I was asked basic questions (what research do I want to do, why Brown, specific questions about the project, questions about background info about me based on what's on my SOP or CV). So all questions you should know the answer to already. I didn't really do anything to prepare for the interview but if you feel nervous try doing a mock interview with someone beforehand. I was very nervous before my WashU Interview because that was my first one and I felt very uncertain about the process but it was straightforward so just try to relax beforehand. They give a timeline of when you can expect to hear back by at the end of the interview. Generally speaking, once you interview that's it. For example, Brown interviews approx 20 people and accepts 8-10. The interview was relatively informal but my PI was pretty chill so it could differ depending on the person. Hope this helps!