 Welcome to English in a minute. In nature, water can be violent and rough, or it can be still and calm. So what could this mean? Still waters run deep. Anna, I'm throwing a party, and you and Ashley are invited. Oh, and Douglas too. Douglas, you mean the guy in IT who never talks to anyone? He's so quiet. Maybe he's just busy thinking. You know what they say, still waters run deep. They also say be careful of the quiet ones. The expression still waters run deep is used when describing someone who seems shy and quiet. But they are often very intelligent and interesting. You just might not know it, because they're so quiet.