Indeed this is one of the risk of over-preparating, and it not only affects FIT but also the Business Case. People who over-prepare end up being rigid and not natural, and losing the "charm" and naturality that one should have in a conversation.
Think of the FIT part of the itnerview as if you were meeting your partner´s parents: you want to impress them, but also be nice, since you don´t want them to think of you as cocky or sharky. It´s the same thing!
Hope it helps!