잔잔한 물이 깊이 흐른다 (조용한 사람이 더 깊은 생각을 품는다)
A quiet or calm person often has a very deep or complex inner character
He rarely speaks in meetings, but when he does, everyone listens — still waters run deep.
Don't underestimate her; still waters run deep, and she's been thinking about this for months.