These are just "Notes to Self¨ for future reference:
A lot of people that I want to draw are happy people, who are grinning widely, even cheesily. If I am sketching a large portrait, then it is easy to draw the grin and the teeth. But if I'm drawing a tiny sketch, like a quick sketch in a sketchbook, a cheesy grin is quite tricky.
When I look at a person grinning, their mouth and their teeth are really visible and clear. But if you actually draw the mouth and the teeth quite clearly, then the person looks very odd (see below). I can't figure out how it is that there lines and tone that we can see on a subject - they are actually there - but when we draw them in a small sketch, they look wrong. I think it is that, when recognising faces in a sketch, our brain wants to fill in some of the lines and tone itself.
So far, I think that when doing small sketches of people who are grinning widely, the trick is to draw just enough that the viewer's brain fills in the rest.
Usually you would draw the upper lip lightly, but reasonably clearly. This seems to be so, even if the woman is wearing lipstick. If you actually draw the lipstick in darkly on both lips, the person will probably look like an evil clown. Usually the upper lip is drawn in because the light is usually coming from above in some way, and the upper lip is therefore in shadow. I think that even if it is not really in much shadow (compared to the lower lip), our brains are really used to this convention, and will interpret it properly.
The edges of the mouth are usually dark, because the cheeks are pulled back. Draw the edges of the grin darkly, but don't let it follow too much down the lower lip.
I think that if the person is grinning so widely that there is a gap between the teeth and lower lip (see above), then you draw a little bit of the darkness, with just the faintest outline of some of the front teeth along the lower edge. Don't completely draw the line between the teeth. I think the viewer's brain knows they are teeth, and fills in the rest itself. The lower lip is not actually drawn too clearly (or somewtimes at all): just draw the shadow which the lower lip makes underneath.
If there is no gap between the person's teeth and the lower lip (see above), I think a faint suggestion of teeth along the upper lip works OK. But again don't draw in the actual lines between the teeth (even though you can see them in the subject). Another way to think of it is that you are drawing the shadows made by the upper lip on the teeth, not the actual teeth lines.