I think there's a few great posts out there that I can attach re: the fit interview. So, in order to avoid not giving credit to others for their posts, please find attach some sources that I recommend viewing. The main takeaway is that the interview process for firms becomes more personalized the further you are into the recruiting process.
On MBB firms in general and their recruiting process
On Bain (vs McKinsey PEI)
How not to answer behavioral questions