Just finished my BCG onsite last month. Virtual but they called it onsite. Four panels back to back with a 15-minute break in the middle. Here's the play-by-play:
Panel 1: Coding, 45 min Algorithm problem with a real-world framing. Mine was about processing a stream of consultant time entries and computing billing aggregates. The underlying structure was a sliding window problem but dressed up in domain language. The interviewer was friendly, gave nudges when I was about to go down a wrong path. I was using Python, no issues with that.
Panel 2: System Design, 60 min Already covered this in another post on this thread but: expect a business-contextualized problem, lots of clarifying questions expected from you, and depth on data isolation.
Panel 3: Behavioral, 45 min This was with a senior engineering manager. More structured than I expected. They had a printed list they were working from (at least it looked that way). Questions around leadership, conflict resolution, and a project I was proud of. They asked follow-up until they had the specific detail they wanted. "What was the outcome?" came up 3 separate times.
Panel 4: Director conversation, 30 min Least technical. More of a cultural fit check and a chance for me to ask questions. The director talked about the team's roadmap for the next 18 months, which was genuinely interesting. One curveball question: "What would you say if a consultant told you your software was slowing them down?" They want to see if you'd get defensive or if you'd treat it as a problem to solve.
Overall vibe: professional, no hazing, interviewers were prepared and on time. The debrief feedback cycle was about 10 days. Offer came with a weekend deadline which felt rushed given the 7-week process to get there. Know what you want before you hit the final round.