Be comfortably prepared with:
1) Why Consulting
2) Why Bain
3) Why this role
You need to have your story ready, and convince them you are passionate + serious about joining Bain AND that you are an impressive candidate that they would want.
Furthermore, make sure you know the company. I.e. What the service ops practice does, the Partners that run it, what some of their recent insights/projects have been, what the job looks like day-to-day, etc.
In Terms of Specific Questions
It's hard to tell you "ask x question". Rather, you need to show that you're engaged, inquisitive, insightful, and curious. Your questions should combine what he/she is saying with insights/knowledge of your own, to then form new hypotheses/questions. Ideally you even prompt their thinking/reflection a bit as well. Basically, your questions/discussion are actually an opportunity for you to show them how you think.
You can ask some quetions specific to the office, but make sure you're not asking silly questions that you should have found out before from some research! Demonstrate your interest, competence, and likeability.