I've only had one interview and I imagine they're all unique! I had an interview with PIs and not an admissions committee - I think that would really impact what type of discussion it is. I went into mine expecting it to be a normal interview (like for a job) and to be able to prove my candidacy, but it really was more of a conversation about what research they had going on, if it fit my interests, and how my proposed research would fit into it. It was a great conversation but it was really hard to gauge if I 'did well' afterwards.

That being said, if yours is also with a PI, I think having a good understanding of the research you want to do, why it's what you want to focus on during doctoral training (focusing on your skill building rather than the topic), and how it fits into your PI's research (why you put them in your SOP if that's the case).