[warning, rambles below]
I can be actually very quiet too you know. Less recently, it depends if I am comfortable or not I suppose. At school I was very very quiet to the extent no one noticed me almost, the advantage in that is people don't realise you've heard them conveying a very dirty secret to a friend.
Quietness would be a learnt side as it was mainly something given by school, when I was moved across the country people took the mick out of my name and so on so I basically isolated my self slowly and eventually as it got pathetic (for most) to take the pick out of someones name they took the mick out of my new personality of, well not having one as I ignore people a lot of the time (as most people who talk to me were bullies). Luckily, and no thanks to my mother or teachers, I was able to identify my problems and prepare to adjust them as soon as I could move to another school (doing so in the same school creates more bullying it seams) so by college I was mostly normal again! Still can be a little quiet when confronted with strange situations when I havent a clue what to do but more often I seam to be adopting a Mr Bean persona for that.
------
"You cannot shake hands with a clenched fist."
Indira Gandhi