What they probably mean by over prepared is a candidate with canned answers or sounding rehearsed. When interviewers ask questions they aren't looking for everyone to give the exact same answer, as most of their questions have a range of good responses with no perfect solution.

A good way to prevent sounding rehearsed is to not try and memorize different TMAAT/ WWYD questions and answers. Instead, really study your own resume/app/logbook/stories so you can draw from your experiences comfortably as it relates to the interview.