still waters run deep

alternative still water runs deep

1. A person with a calm appearance has, or may have, considerable inner emotion, character, or intellect.

   I always knew she was an artful jade; 'still waters run deep;' but she shall be exposed, the mask shall be stripped from the hypocrite.

   1822, Catharine Maria Sedgwick, A New-England Tale

   3 more examples

   But still waters run deep; and no crisis had come in the years of her early maidenhood to demonstrate what lay hidden within her, like metal in a mine.

   1885, Thomas Hardy, A Mere Interlude

   Baxter had never known of any goings on in that quarter, but, bless you, still waters run deep, and these girls were all alike, one as bad as the other.

   1903, Samuel Butler, chapter 58, in The Way of All Flesh

   Isabel Pierce, the central character of Sweetwater, Roxana Robinson's fluid third novel, gives the appearance of being a thoughtful, reserved, quiet woman who won't rock any boats in her life. Yet she harbors passions; it might be said of her that still waters run deep.

   2003 June 8, Alice Elliott Dark, “Book Review: Rainy Day Woman”, in New York Times, retrieved 09 Jul 2008