No kidding. Every time I hear someone is "so quiet," I go straight to DefCon 2.
Nobody ever says, "Oh...we saw this coming from a mile away; he was a loud-mouthed a-hole" after a guy goes on a killing spree. No...it's always "We're shocked. He was always so quiet."
No its just that they are "shocked" when the quiet guy goes (excuse the expresion, postal), they are NOT shocked when the a@@hole goes postal....