"A portrait is a painting with something a little wrong with the mouth."
John Singer Sargent (1856-1925), prolific American portrait painter
To continue my free portrait painting lessons for those preparing for "Catherine, Duchess of Cambridge" portrait challenge I am now studying the mouth. The mouth is extremely versatile; we us it for kissing, eating, smiling, frowning, talking ... and the list goes on. Mouths are never at rest, not even when we sleep, and all that exercise builds muscle. These muscles pull and push the mouth into an almost infinite number of expressions. The tiniest change in light, shadow, colour etc. will change the expression, which accounts for the above quote.
I think painting Catherine's mouth is going to be the most difficult part of the portrait. The Duchess has a wide, fairly thin lipped mouth but she is usually smiling. You rarely see her with her mouth closed because she is either talking to someone or showing her lovely white teeth through a smile. Try and Google "portraits" to count the number of people that are painted with their mouth open ... it's next to zero! Painting a wide open mouth is tough to pull off, because it is difficult to paint natural looking teeth. I've heard the word "tombstone" to describe the effect. In his interview, Paul Emsley mentioned that he had to change the mouth in his portrait, so he probably had difficulty with it too.
Therefore, if you are a beginner I would recommend keeping the mouth closed and aim for an enigmatic smile like the Mona Lisa. But if you're brave, an open mouthed broad smile will give your portrait a more natural and contemporary look.
Here are some links to wonderful videos on painting the mouth which may help.
The Mouth - Wet Canvas
Dimensions of the Human Form
How to paint the mouth or lips front view - Art Lesson Videos
How to paint the mouth Part II - Lance Porter