Often when I'm drawing faces I find myself mimicking the expression that I'm trying to convey. Like today I did a string of roughs with Mrs. Otterloop going from nonplussed to dawning comprehension to full awareness, and my face kept going from slack to alert in sympathetic response. Nothing too hammy, just enough that I could feel it while I drew it. Animators do it purposefully, and even keep a mirror handy so they can model for their drawings. I once read a hilarious account of Goofy's animator jumping out of his chair and lurching around the studio with that slap-footed Goofy walk, working his adam's apple and everything, then throwing himself back in the chair and drawing what he'd just done. It's the same process that kids use when they make kkkkapppcccccccchhhh noises when drawing battleground explosions. And I'm sure fine artists do it in the privacy of their studio. I'll bet Picasso yanked his face around to the point of malocclusion when he was in his cubist period. His girlfriend of the moment probably told him to quit it before his face stuck like that, too.
