Had my TBD last week.
My advice is simple: Relax.
This interview is likely not going to make or break your admission decision. I spoke to more than one current student who said they thought they didn't do well in the TBD and obviously still got admitted. You don't need to dominate the conversation or talk a lot, just be friendly and helpful to the group rather than try to "show off".
Just my opinion from talking to current students and committee members: It's not Pass/fail. It's just another snapshot to add to your overall application. If you're a "borderline" candidate to begin with, perhaps an amazing performance might tip the scales in your favor, but this isn't true for everyone. Your GPA/GMAT/WE/Recs, etc matter more than the interview, I think it's something they use to get a better snapshot of who you are, and verify that you're not a jerk, can't speak under pressure, etc. 80%+ of people I imagine are going to have a perfectly fine TBD and their admit/deny decision is going to hinge more on the rest of their app than the TBD.
Just my thoughts. Good luck everyone!