The interview itself was very quick and straightforward--the easiest part of the process. We arrived early and the interview was over before our scheduled time. They give you a short form to fill out and a list with the order of documents when you arrive, and we wished we had had that ahead of time--that list needs to be available online in an easy place to find. But everything was fine in the end and our K-1 visa was approved! The interview happens at a window, standing up. The consular officer, a woman in her thirties, asked two questions of my fiancee: how did you meet your fiancee, and have you ever been married (and asked also if the petitioner had been married). She said "I'm going to cancel your tourist visa now", and took out a big stamp and cancelled it. We knew at that point that the visa was approved. She then said "your visa is approved and will arrive in (I can't remember how many) business days".