John, for instance, spends most of his time with a sort of smug leer on his face; that's because the man thinks of himself far too highly. We see a destructive, petty, self-centered, domineering, entitled and heartless dingbat who crashes through life not caring about all the damage he does and not realizing that the people around him might actually be in the right. We thus see him smiling an inappropriately indulgent smile at the people his blinkered vision sees as just not getting it.
Mike too has a certain specimen facial reaction: the face of someone who thinks that he's being unfairly treated when he in fact isn't. Since his every thought rotates around the need to deflect blame from himself to others and since he thinks that people are trying to humiliate him and keep him down forever, the facial reaction I think of when I think of him is a sullen frown.
Even April has a set reaction: the face of someone who deep down knows that she's not liked much and more or less has to suck it up. It hurts to be called a picky-face Martian princess but it's all she knows.