Silent Cal was a dick who advocated trickle down tax cuts about a century ago, but this anecdote applies to the topic.
[quoteinvestigator.com]
"I'm working out some complicated math in my head. You wouldn't understand."
When silence is worth gold, words are worth silver.
I'm anti social, I hate humans, or I speak when spoken to, it's your call
I haven't been asked this question for ages, now. I used to be quiet because I was depressed and because before that people used to whinge about me being too loud. Damned if I did, damned if I didn't. So now I'm at whatever volume I want to be and if people don't like it they can cram their opinions back up their ass.
Just shrug and say nothing. That would seem the most characteristic thing for a quiet person to do.
It's usually because I am thinking and I say so. I like some of the responses given by others though.