hey there. I had something similar for CMU PSN this week - 4 one-on-one interviews with faculty I'd nominated before. had half-an-hour interviews. these being my first grad school interviews I also didn't know what to expect. Well they were very cordial and talked to me like a scientist talking to another scientist. They asked me what I'd done research wise before this. Then they talked to me about their research plans. That was it, I could ask more questions about the program, university, city, etc. if i wanted. encouraging experience overall!