So, I will share my experience with you, maybe it will help. I had an interview at Berkeley. The interview was conducted by two professors, one of whom I had previously spoken to. I didn't know if I should be prepared for a formal or more casual interview, so I studied and was ready for either one. The interview itself was a mix of formal and casual, some of the questions were almost like a conversation. But then, they asked me more dense questions, like which archives I believed I could research during my Ph.D., and how I was planning to link my object of study to a transnational approach. I answered everything and they seemed to have liked my answers. Maybe you could prepare yourself for something like that...Try to imagine what troubles you may encounter during your PhD (sources, archives, methodological questions, etc).