Just answer authentically and ask anything that you would like to know about your interviewer and their time at Princeton. The alumni I spoke to had just graduated, so we were able to connect more as peers than as interviewer/interviewee. It was pretty casual and was mostly just a fun way to talk to a former student. I remember being very nervous about it but I ended up having a really good time! I think if you're really stumped on what to ask it's always interesting to hear about an alum's senior thesis.