Ask about them
People love to talk about themselves. Just ask them what they've worked on recently or what their journey at BCG has been like. Importantly, do your research beforehand - find out what they've done and try and pick something from their past that looks interesting to you. Tailor your questions just like they tailor their FIT questions to your resume."So, how did you end up in the Industrial Goods sector?"
Ask insightful questions
Ideally, your questions should also show them that you're insightful, inquisitive, bright, thinking about current topics, etc. For example:
"I saw you worked on a big digital transformation for a financial services company - I experienced one myself and it seemed so hard to move such a big organisation. What did you see as the major challenges and breakthrough moments?"
"I know that x big trend is happening. I also know that y types of companies exhibit z traits. How do you reconcile this difference when ensuring y companies adopt x trends?
Summary: Ask about THEM and ask insightful questions!