Still waters run deep

1. A person's calm exterior often conceals great depths of character, just as the deepest streams can have the smoothest surfaces.

Idioms and Phrases

A quiet person may be very profound, as in Susie rarely says much, but still waters run deep . The physical observation in this term dates from ancient times, but it has been used figuratively since about 1400. Anthony Trollope amplified it in He Knew He Was Right (1869): “That's what I call still water. She runs deep enough. . . . So quiet, but so—clever.”