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It depends on what you want to get out of the meetings. I treated each meeting as a chance to get enough information to decide whether to rotate with this PI. So I tried to structure the conversation toward things I care about, which does include their specific research approaches and future directions. But I think it would be a missed opportunity for you to only consider research. You should also try to get a sense of, "does this person's personality/work style fits with mine?" which is admittedly difficult in a brief conversation. You can straight up ask the question of "What's your mentoring style?" or "How often do you meet with your students?" Check out Quora and Academia StackExchange for much better answers than mine. For example: What are the 'right questions' to ask professors at a visit day for prospective graduate students?For people that have already been on 2nd looks, how much "background research" did you do beforehand on each of the PIs that you met with? Just enough so that you could ask them relevant questions about their research? Or read one of their most recent publications so you can can really get into it and have specific questions?