Two rounds: first was on-campus, not office specific, with much younger members of the firm. Bain's cases follow a very structured model, for which Profit = Revenue - Cost almost always work. Most cases start with some brainstorming, moves on to basic market sizing scenario, and closes with some calculation before you present ideas. As there is a meet and greet session the night before the interview (at least at my school), there was no time wasted on fit or questions on leadership experiences and such. ~30 minutes per case.
Second round will typically be at the office (although, with some offices, interviews are held at the closest local office - perhaps to save the company some money?). ~45 minutes per interview. Two interviews, with higher ranking officials; usually at least one partner, but not always. Cases are *very* similar to first rounds, but are a little longer. Interview is longer as around 10 minutes is spent on fit questions (why Bain/consulting, what you enjoy doing on campus, describe yourself in three words etc). I don't recall getting an questions on leadership experiences or "how did/will you handle x scenario" type questions either.
Be yourself! Bain loves people who are charismatic and engaging, and this is typified in the people you will be interviewing with. Of the three (MBB), in my experience, Bain had the easiest, most straight-forward case, with a clear path and relatively easy and direct numbers.