“Still waters run deep” is used to describe people who are quiet and tranquil. These people often have “deep,” sophisticated personalities. Even if someone doesn’t talk a lot, he/she could still be extremely thoughtful. When the surface of water is rough and fast, it usually means that it is shallow, like in a river or stream. But water that is calm and still is often as deep as in a lake.
Who among us do you think belongs to this noble category of people about whom you could say "still waters run deep"?