Do you think the blank, expressionless eyes stereotype could be down to confirmation bias? I'm not saying the stereotype is necessarily wrong (stereotypes do come from somewhere, after all), but I'm sure many non-sociopaths also have vacant expressions that we aren't noticing because we aren't scrutinising them.
(Gethin is from the UK, so he spelled "scrutinizing" correctly.)
This is something I've thought about a fair amount, so it's worth a post of its own. I'll rephrase and expand on my reply to Gethin here.
First of all, yes, there's definitely the possibility of confirmation bias here; there always is, in such situations. But, as Gethin suggests, the stereotype exists for a reason.
When I first heard people talk about the "blank, empty eyes" of a sociopath, I thought, how ridiculous. Eyeballs themselves are incapable of any expression other than changing pupil size as a possible indicator of interest, but most people don't even notice that.
I soon realized that what people were referring to was the way the skin around the eyes gives the "eyes" their expression. A smile or frown can often seem to extend to the eyes. "Narrowed eyes," i.e., tensed eyelids, make one appear angry, and so on.
What gives the impression of "blank eyes" are an absence of an expression. The absence of an expression, or complete relaxation of the muscles of the face, is indicative of a complete lack of stress. So it makes sense that sociopaths would often have a blank, stress-free expression when with other people, since they have no regard for or concern about others.
Think of how you feel most of the time you're conversing with other people: you're at least a little concerned about what they think of you, the impression you're making, whether you're being accurate in your statements, and how you're making them feel. And your face will reflect whatever tension or stress you feel, giving your "eyes" expression.
Another thing stress can induce is watering of the eyes, which can help make someone's overall expression appear more heartfelt (for good reason). It can also indicate amusement, a la "twinkling eyes." (Or even, "I laughed so hard I cried.")
A sociopath feels no such tension, since they regard you or me the same way we would regard a bicycle: as a tool to be used. Ergo, no expression. (I've seen this up close, and been struck by it.)
Now think of the way your face feels when you're watching a somewhat boring, late night rerun on TV which is gradually lulling you to sleep. You feel zero social stress, so you're completely relaxed, and are wearing no expression. At that moment, you, too, have the "blank, empty eyes" of a sociopath.
The difference is, that is not the expression you wear for social situations.
Having said all this, I should also point out that sociopaths are usually consummate actors, and can often feign emotions so convincingly that they appear more heartfelt than those of non-sociopaths who are actually feeling those emotions.
But, if you know what to look for, sociopaths will also always give themselves away in various ways, and one of those ways is that they sometimes wear a completely blank, empty expression.