What should I ask a potential PhD advisor in an informal interview? I emailed a professor at a school I am applying to, outlining my research interests, two specific projects proposals, and how they align with his research. I let him know that I am applying and I'd be interested in working together and chatting if he's taking students. He said he is taking students and asked if I'd like to set up some time to talk.
What do I talk about?
Here are the main topics regarding his research I can think of:

How did you come to your line of research?
What topics do you feel are under-explored in our sub-area?
What projects are you most actively working on now?

Should I also ask for feedback on my proposals, or would that sound stupid?
To clarify, I closed my email with something like: If you are looking for graduate students and feel our interests align, I'd like to discuss opportunities for working together.
Actually, your questions sounds like you are trying to hire him for a position, whereas he would be the one taking on an obligation to you and your future. I would probably have a chuckle if a student asked me those in a first interview. The first question would be good over coffee after we'd worked together for a while. For the last question you might first find a couple of his recent papers rather than ask.
But the more important questions are things like

Can you help me find a topic that interests both of us? Follow up on this question if some suggestions are made.

How independent do I need to be (do you expect me to be) and are you available to help if needed?

How successful have other students of yours have been in completion and in their careers?

Is there a seminar in which you and grad students share ideas?


If you already have a topic that you want to work on then:

Do you think ABC is a fruitful line of enquiry and (if so) can you help me explore it?

But approach such questions cautiously. Some will take offense. Be prepared to be the answerer of questions, not the asker.