In novels, characters are often defined by their physical appearance. The man who frowns all the time is negative and irritable; the woman whose conscience is clear shows it in her relaxed demeanor. It's a useful tool, and it may often be true that our natures are betrayed in physical manifestation. But I don't think that's always the case.
I direct a choir of some fifty people, and if their expressions while singing were used as a judgment of their mental state, one would conclude that they were all in pain. One woman in particular whom I don't know very well seems, judging by her expression, disapproving of my leadership and the group as a whole. It was only when her daughter told me recently that she is thrilled with the choir and eagerly looks forward to practice each week that I learned differently.
Detectives in real life probably take note of the fact that concentration and concern often appear on one's face like anger or disapproval. I'm not immune. Often when I was teaching, students would confide with relief after they'd been in my classroom for a while that they'd been apprehensive about me. Despite the fact that I'm a pretty happy person, I tend to frown a lot when not engaged in conversation. It may be from being focused on what's next, or maybe it's simply my astigmatism.
Of course, in fiction, we need to have characters reveal themselves, so we toss in physical clues. If real detectives drew conclusions like fictional ones do, based on the briefest of expressions or the tiniest of idiosyncratic tics, I suspect we'd have people arrested for something diabolical when in fact they're guilty only of dyspepsia.