I attend a small liberal arts college in the northeast that is not a core recruiting school for Bain, but Bain has started to do info sessions on our campus and look at more of our applicants. I submitted an application through an online college career center job board. I think they probably selected people for first round interviews solely based on the highest GPAs in the applicant pool. Through this recruiting channel I believe they selected 4 people for first-round interviews through this process out of maybe 80 applicants.
First round interviews were over the phone, two 30 minute back-to-back case interviews. Most Bain cases are the same, so I'm just going to lay out the basic structure they follow rather than go into the details: first they ask for factors or buckets of factors that go into your initial thinking on the problem. Then there's a market sizing question. Next, a break-even, profit calculation, or larger math portion of the case. There is always a twist in the math, so write out your equations carefully. Finally, a "curveball" qualitative question where the interviewer is looking for you to be creative.
Second Round Interview was a similar format of two 45-minute case interviews back-to-back but in-person at the Boston office. Similar case format as the first round cases, but slightly more involved.
Everyone who worked as an AC, Consultant, etc. at Bain was really nice and extremely helpful. However, HR and the people responsible for scheduling the interviews were a mess. Inconsistent e-mails, no clear details, etc. Also, if you go on to the next round they usually call you 2 days after the interview, and if you don't go on to the next round (or don't get an offer) they usually get back to you at the end of 2 weeks.