First round - two interviews with Principal / project managers
Final Round - 3 interviews, One Partner, One Principal (a partner cancelled), then one Senior Partner
Interview process is really well handled by their HR team, very impressive in how they guide you through it. Details given along each step, and I would really recommend the pizza evening they offer before the first round to get to know some of the case study process, really helpful for someone from outside consulting. You can use case frameworks to a degree, but the reality is that you need to use intuition, they put you off balance very often. And don't be surprised to meet a bad cop style interview where they press you a bit, especially in the final round.
All interviewers were very as you'd expect until the final round, when I had an interview with Eric Ellul. He really put me off the firm. He's a very senior guy, didn't really seem to pay any attention (forgot details of the case he was setting out and changed variables after he took a phone call for a few minutes, he clearly forgot what he'd set it out as) but it just really disappointed me compared to the rest of the culture of the firm. The interview was meant to be 45 minutes, but I was out the door in less than 12, he didn't really give me a chance to get into the case at all. When the interview was over and I was leaving the floor I saw him bark at one of his subordinates about a report. It really surprised me, BCG from what I had seen in meeting many people before that was not that kind of firm, but I started to doubt if I was wrong or not. It seemed to juxtapose the general care and culture of the rest of the firm. I thought it would be a great place to work, but I really doubted myself afterwards. I don't think I'd apply again. If what I saw there was the examples of the upper echelons, I don't think its a culture you would want to experience day-in-day-out.