micro expressions that show on faces before the 'wearer' checks themselves, are often noticeable and show the true emotion.
the plastered on smile/grimace, the arms folded, the stiff body language, the silences
we read faces and unfortunately for some people their face is such that it looks like they are having an emotion that they are actually not - such as a down-turned mouth, frown, angled eyes, wrinkles etc and so they appear grumpy, angry, sad, emotional etc
i have known a couple of people who have had a very definite issue like this.
i have noticed it with actors too - the ones that always seem like they're about to cry, or smirk etc regardless of whats happening and sometimes i have found it hard to watch the show, as i am misinterpreting the emotion of the storyline and character.
nerves and shyness are also often perceived as rudeness