Still waters run deep

Still waters run deep - quiet people are often very thoughtful.

Example - "- I get the impression Nathan is not very smart. He never says anything.
                   - Don't be so sure. Still waters run deep, you know?"

Image - https://s-media-cache-ak0.pinimg.com/564x/fa/d2/d8/fad2d82aa46135e0763005b531d19f4e.jpg