“What started as a great interview became mediocre (at best) when the participant asked a simple question… and my client answered.

The problem wasn’t the question, it was the way it was answered.

You see, my client gave a thorough answer that showed his expertise.”

I see this often, and am guilty of it myself. You have to fight the natural urge to try to 'impress' the customer with how smart you are and how you have all these ideas for how to make your product better; because if you do 'impress' the customer, you might also intimidate them. If you show how knowledgeable you are, they won't want to embarrass themselves by revealing that they know less than you do. This means that they are likely to simply agree with what you are saying, rather than answer your questions honestly, and so the whole thing will be a waste of time.