Completely agree! I always seem to do better when I have 'prepared' less. Obviously do still need to know enough about the firm, but when I don't prep my answers in advance, I always do better. I think it is because ultimately an interview is just as much about being able to answer the question as it is about thinking on the spot. If you're reciting pre-rehearsed answers, I think the interviewers can tell you are not really thinking, just reciting pre-memorised content. Plus, sometimes you respond with a pre-rehearsed answer that you think is answering their question, but really isn't.