Brief thought exercise: where's the line in a job interview between being casual/funny/personable and the definition of professionalism?

[youtube http://www.youtube.com/watch?v=b56eAUCTLok&w=620&h=315]

Feel like this has happened to me a couple of times in the last six-seven months: I'm at a job interview, and it's one of those situations where you meet with 3-5 people in a given day, for about 30 mins-1 hour per. I actually somewhat like this format, although by the end you're tired of t…