Did my interview yesterday. The TBD is very relaxed. There is NO need to stress over this. I literally prepped my 'idea' in the five minutes before the discussion. You have 1 minute or so to present your idea, so nothing complex. The rest is just about being a friendly teammate and contributing to the ongoing conversation. If you stress about this, you are going to hurt yourself way more than help yourself.
The 1 on 1 interview is VERY short. You are always asked "what role did you play in the TBD?" and then "how does that compare to the role you typically play?". Then there is room for one more question, which for me was "Why Wharton?". Keep in mind that these are with 2nd year students, so not high pressure and probably not huge influence on the admissions decision.