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?" "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?"
Summary: Ask about THEM