Not sure about in-person interviews, but I can answer regarding the unstructured interview portion. They ask 3 structured questions that are usually ethics- and service-related (e.g., one of mine was about organ transplants). For the unstructured, they can really talk about anything, but they're blind to your application, so they may sound more like the structured questions (for example, one of my "unstructured" questions was about how I felt about the US healthcare system). They typically ask at least 2-3 of these.
Regarding questions you should prepare, I would have at least 2 questions to ask. I didn't get to ask any since they asked if I had any questions right at the 25 minute mark, but it would be a good idea to have some in case you answer quickly.