The saying 'still waters run deep' came to mind. Here's something written about it on the internet. I'm probably waffling off topic here but I thought it was quite interesting.


To describe someone with the phrase still waters run deep may mean that the person may appear calm on the outside, but be seething with passion underneath their cool exterior. Still waters run deep may also mean the person appears to have nothing interesting to say, but in fact, is a deep thinker and has many ideas. The proverb still waters run deep may be traced to Rome in the first century. In Quintus Rufus Curtius’ work Alexander the Great, he writes: “altissima quaeque flumina minimo sono labi”, which translates as “the deepest rivers flow with the least sound”. The proverb was written as an Aesop fable that was included in the collection published by Roger L’Estrange in the 1690s as A Country-man and a River.

''Seething with passion."