Three rounds of two interviews each (= 6 interviews in total): First round with consultants, second one with managers and/or principals, and last one with partners.
A case is given every time by the interviewer, mostly taken from his own experience and adapted for the interview. Make sure that you understand the specificities of the case and adapt your framework to the company's case.
The interviewers are mostly very approachable and open, which reflects Bain's culture at all levels of the recruitment process.
One of the interviewers takes time to provide the candidate with a constructive feedback systematically a few days after a round of interviews. Always take into account the feedback given and work on it before the next round of interviews, as they expect you to have improved on the weak points spotted early on.