Still waters run deep
Definition 1 of 1
Pro Tip 1/3Misperceived Quietness
The proverb implies quiet people may have rich inner lives, not that they are uninteresting or simple. 
He doesn't talk much at meetings, but still waters run deep—he's actually a brilliant strategist.
Pro Tip 2/3Non-Verbal Insight
Observe non-verbal cues with quiet individuals; their actions might reveal more than words. 
She silently donated her winnings to charity—still waters run deep, indeed.
Pro Tip 3/3Contextual Use
Use the phrase to express respect or admiration for someone's unseen qualities. 
He's the calm one in crisis situations—remember, still waters run deep.