My uncle told me about a Boeing interview process one time. They would put candidates in a room together and give them an engineering task/problem to solve. At this point, they'd already vetted them for knowledge/competence.
At first, the candidates would talk and do the chit-chat thing, who's who, background, blah blah blah. But at some point, one would push back his chair or pick up the notebook with the assignment or get up to the whiteboard and say something like "Well, let's get on with this thing."
That was typically the end of the interview. Of course they'd still observe the candidates as they worked on the project, but that bit of leadership was primarily what they were looking for.
Because of this, and stories like it (such as TFS), I intentionally go to interviews unprepared. My intention is to present them with the "real" me that they are more than likely to get. I don't want to present the best of me, because they won't get that all the time. If asked why I am not more prepared, I will answer with exactly this. One could argue that this IS being prepared...
I have had only one interview out of my last 7 (spanning the same number of years) that didn't return a job offer.