A reticent person is reserved, not inclined to speak freely or often, disposed to keep quiet. A taciturn person is also inclined to silence and reluctant to take part in conversation. However, a taciturn individual is usually considered surly, dour, sullen, and severe. That is, a reticent person is pleasantly shy, withdrawn, and silent; a taciturn individual is unpleasantly so. Related words are silent, uncommunicative, reserved, still, and quiescent.
Dictionary of problem words and expressions. Harry Shaw. 1975.