Well, that's hardly a novel activity! Millions already have, and millions will in the future, whether the original painting stays up or crumbles to dust. Her smile has been familiarized almost to the point of boredom, and yet, while flipping though pics of old master works for study reference I thought... why not study her? So here's that sketch, and afterwards I made those graphics while responding to a query by a friend.
I realized that Leo has used a
set of three curves, arranged in groups to get that particular expression. I found the same mechanism, albeit much less obvious in Dominique Ingres'
Mademoiselle Riviere. Mona Lisa has a softer, melting expression, add to that the effect of the incident light, which makes the 'curves' more obvious in her case.
pic 1.
pic 2. The 'curves'...
pic 3. The areas affected emphasized