I had my off-campus interview on Friday. I think I had a similar experience to most people. It was a very casual conversation. I wasn't too nervous but the alum I had made me feel comfortable right off the bat. He didn't actually ask too many questions. He told me about himself then asked me to walk him through my resume. I think in doing so, I was able to answer a lot of the basic questions about leadership, motivating a team, etc. He asked the why MBA now, why Kellogg, and what are your future career plans questions all at once. Having Essay 1 done certainly helped with that! He then took the time to give me some unsolicited, but thoughtful advice on my future plans.
At the end, I asked what the process was for him after the interview. He actually showed me (very quickly!) the form they have to fill out. To visualize, think of the form recommenders have to fill out. Starts with a grid to rank the applicant on leadership, communication, etc. Then there are open ended questions on similar traits. One asked about maturity and another asked if the applicant seemed to be listening. I also think there was a question about the applicant's future goals; are they clear, etc.
It lasted 50 mins. I feel like it went well, but wish I had more time to work in some other things I was hoping too.