Snape makes a lot of distinct facial expressions, and never ventures out of that realm. If he were to smile, it would disrupt the Space-Time Continuum. Or the Snape-Time Continuum.
Calm
When nothing in particular is bothering Snape, he still looks quite perturbed... because he is. Something's always annoying him, and he's got horrific memories engraved in his mind. Keep your lips pursed, but only slightly. You don't want to look like you're always about to kiss someone. Your brow should be furrowed, but not so much that you really seem mad. Your eyes should be half-closed, but relaxed, not squinted. But even when they're calm, they should be able to strike fear into the hearts of many. They should be cold, and remind you of dark tunnels. Now, Snape is using this expression most of the time, as he hides his true emotions... That is, unless he needs to get a point across. This is his calm face:
Angry
Ahh, my favorite Snape emotion! He's the kind of guy that makes you want to get him angry, because it's sexy. But to the people he's angry at, namely students, it's also quite scary. For Snape to actually show that he's angry and not hide it, he has to be pretty angry. At the least, you should snarl up your lip, but try not to show your teeth. In some instances, anger can coincide with just being extremely annoyed, in which he's still pursing his lips, but sort of sucking inward. It's hard to explain, so here's his extremely annoyed face:
Anyway, his scowl is legendary. You only see it when he's really mad, and it's important you get it perfect. Furrow your brow as far as it can go, and force the edges of your bottom lip downward. Your eyes should only be slightly more open than normally, but others need to see the anger in them. They're peircing, scrutinizing, and freaking scary. Also, flare your nostrils.
Sad
We hardly ever see this emotion out of him, and I'm glad we don't, because the sight of Snape full of sorrow is absolutely heartbreaking. Obviously, if you're trying to be like Snape, you're never going to show if you're sad in public... unless it's impossible not to. When Snape is sad, or is struck with horrible memories that haunt him, his eyes'll get visibly softer, and he'll relax every muscle on his face, even his lips. This is the farthest it'll go if there are people around, but if it's bad enough and he's alone, it's almost as frightening as seeing him angry. Every muscle on your face would be strained, and your mouth would be contorted into a horrible, agonizing shape. It hurts to describe this, so here's pictures.
Sort-of sad:
But we've never seen him actually crying, so no picture for that.
Happy
Despite what you may have heard, Snape can be happy... but evilly happy. However, he doesn't express it normally. For him, a smile is unfurrowing his brow and softening his eyes a bit, and pushing his cheekbones outward and upward. But still, your lips are pursed. It feels like you're smiling, but others can't see it. If you feel deliciously evil, you can grin out of one side of your mouth. Snape's grin:
It sort of creeps you out... doesn't it? But remember, he would never actually laugh. He would also never let anyone see a genuine smile... You should never genuinely smile unless thinking of a certain person you're in love with that died... or they could still be alive, whatever.
Okay, and I know you don't choose what face you're born with, but this is a lot easier if you have thickly defined lips and dark eyes... and a huge nose. It just helps. Each and every expression he makes has a much bigger meaning behind it, and it would be impossible to explain it all in one blog. So, I suggest you watch the movies reapeatedly until you've perfected it.
