Applications are office specific, make sure to attend recruiting events. Referrals from existing or alumni Bain employees are highly regarded - so have your classmates put in a good word (especially school reps). Initial round was on campus: included case interviews with 2 interviewers (not office specific). Next round was at a different school, another 2 interviews, usually from the office you applied to. Decisions are made fairly quickly after each round of interviews. Interviews are usually 45 - 60 min long, with the first 10 - 15 on fit and the remainder for the case.
I applied online and got a online assessment after ~1 week. I completed the online test but I haven't heard back since then. The timeline is a bit uncertain to me.
First round case interviews. Got standard questions about profitability and market entry. IMO the most difficult part was reading complex charts and distilling the key insights with time pressure. There was little behavioral.
The interview was overall efficiently run and very professional. It had 2 stages; a case interview typical of management consuting interviews and a fit interview to assess cultural fit with the firm.