I have read that a good way to determine whether a person is introverted or extraverted is how he or she copes with stress. Extraverts tend to go to other people, do physical activity, or try to change their environment. Introverts tend to retreat into themselves, contemplate, and change themselves. I'd have to say my way of handling stress is definitely the extraverted style.
I do tend to be pretty reserved at times, though; but I can also be very talkative once I get started—as I think most everyone has found out. Everyone is a mix. NTs have typically found me to be too shy, tense, reserved, and such; aspies tend to find me too intense and overwhelming. It's odd.