I have seen quite a few places where a smile plays about one's lips comes up
When Elise saw these things that were so dear to her heart, a smile played about her lips and the blood came back to her cheeks at the thought of being able to save her brothers.
She leaned back, her eyes closed and a light smile played about her lips.
This page has plenty of examples from real-life sources, and so does Google Books.
I can't understand why the preposition "about" appears here, instead of "on." A smile, as far as I understand, should play on one's lip, and play about means to play around. Am I wrong?
Best Answer
A smile involves more than one's lips --- it includes the whole face. In fact, the lips are often the last facial feature involved. So a smile playing about one's lips is a smile happening around the mouth, and probably starting to affect the mouth. It's the beginning of a smile, a smile threatening to break full on to someone's face.