Really interesting article about facial expressions, and some folks who've done a huge amount of research on them:

"Ekman then began to layer one action unit on top of another, in order to compose the more complicated facial expressions that we generally recognize as emotions. Happiness, for instance, is essentially A.U. six and twelve -- contracting the muscles that raise the cheek (orbicularis oculi, pars orbitalis) in combination with the zygomatic major, which pulls up the corners of the lips. Fear is A.U. one, two and four, or, more fully, one, two, four, five, and twenty, with or without action units twenty-five, twenty-six, or twenty-seven. That is: the inner brow raiser (frontalis, pars medialis) plus the outer brow raiser (frontalis, pars lateralis) plus the brow-lowering depressor supercilli plus the levator palpebrae superioris (which raises the upper lid), plus the risorius (which stretches the lips), the parting of the lips (depressor labii), and the masseter (which drops the jaw). Disgust? That's mostly A.U. nine, the wrinkling of the nose (levator labii superioris, alaeque nasi), but it can sometimes be ten, and in either case may be combined with A.U. fifteen or sixteen or seventeen."