Explanation:

Still waters run deep.
Still waters run deep is a proverb of Latin origin now commonly taken to mean that a placid exterior hides a passionate or subtle nature. 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.