Still waters run deep

Proverbial Meaning: People with a quiet manner may often have complex, deep inner-thoughts and understanding. On the other hand, people who talk a lot and brag about how much they know will often be less intelligent or more shallow of thought.

Literal Origins: Streams where the water is deep don't make any noise and often appear still on the surface, but in reality a lot of water is being moved beneath the surface. Shallow streams bubble and jump over rocks, making a lot of noise but transport only a small amount of water.

Common Usage: This proverb is extremely well-known in the English language and is often used to describe a person who is not outspoken but has shown themselves capable of achieving or doing something extraordinary. The proverb can have both positive and negative connotations, depending on the nature of the person being described.

Conversation examples could be:

"That new guy Paul is very quiet isn't he?"

"Yes, but they do say still waters run deep."

People may also use this proverb when someone they know who is normally quiet and shy is found out to have been doing something unexpected, which could be practically anything from being a secret millionaire funding orphans in Africa to being a serial killer... or perhaps they've been secretly leading a  promiscuous sexual lifestyle.

"My God, did you hear about Sarah running off with the boss? It must have been going on for months, she's so quiet too!"

"Well they do say still waters run deep."

In fact the proverb is so well known that some people may have simply answered: "Still waters." and exchanged knowing glances, possibly with a smirk in this instance.