Post-doc interviews can vary quite a bit. When I was looking for a post-doc I was in the running for three. For one, because it was international, we were planning on a meeting at a major conference. However, I withdrew my app before it got to that point. For the second I did a phone interview, and was in the process of scheduling an on campus interview, when the post-doc I took came through. That was a training grant, so once I was accepted through the program there were no formal interviews, but they evaluated my CV, letters, and research proposal, and I had met with the PI previously. Ultimately, that ended up being a poor fit (there were a lot of factors that went into the decision to take it), and I almost immediately started the hunt for something else. I came into my current post-doc by striking up a collaboration with my new mentor, and then once funding was available, I was hired so there was no interview (I also then got my own funding). This is probably not the most "normal" scenario. However, in my old lab, my PI made it a habit of flying in all post-doc candidates. They gave a talk, met with the PI and other faculty, along with current grad students and post-docs. This allowed everyone to gauge fit, and expertise/skills. I also have several colleagues that did so on their post-doc hunt (and would have for at least one of my applications). It was structured similarly to a faculty interview, but at least in the cases with my old lab, it was much more of "I like you. Let's just make sure this works". This is likely similar to anon's experience, such that nobody wants to get burned. Good luck with your search and interviews!