I found all my interviews to be much easier than my fears. Very minimal "on the spot" questions. That one question won't make you or break you probably so don't let it freak you out. All good advice here though, short, sweet, relevant, and try to make it MEMORABLE if you can, like you were once shot out of a cannon or something, maybe even an appropriate joke to end off.
Either way, I find my class to be a usual microcosm of any handful of 60 people. Some introverted, some extroverted, some funny, some with sticks up their butt, all different kinds of styles are represented. There can't be just one way to do an interview, but don't try to be too different than you are in real life or it will come off insincere.
Also, if you're not nervous then you just don't care... they expect it.