There were two rounds of interviews on campus -- each round (one per day) was 2 interviews (30 minutes) back to back, so 4 interviews total. The greeter called to extend an offer the evening after the 2nd round, so the whole process only took 3 days (prenight, round 1, round 2). Generally the process is to have the first round of interviews on campus, and then the second round at the office, but they chose to do an interview "blitz" so both rounds were on campus. People who received offers were then invited for an office visit, which consisted of presentations and "discussion time" with some of the professionals.
Each interviewer seems to have a target for their questioning (e.g., motivation, teamwork, leadership, conflict, etc.) The interviewers can be intimidating at times and the questions are somewhat challenging because they might give you a situation and ask you for parallel situations that you have experienced. They want to see if you have experiences that relate to the ones you may face at the firm.