Today I saw it again. Not in real life, that is: I read a novel where the author had treated the characters badly.
You have a young man, mid-teens, who the author has gone to great lengths to impress you with his intelligence, his cynicism, his general streetsmartness. We’re not talking ordinary intelligent, either: he has already on a couple of occasions outwitted several university professors.
Then, out of the blue, he acts like a complete and utter moron. For no apparent reason, he decides to completely trust a person who he has, not thirty seconds ago, given good reasons to hate him. On this person’s advice, he then proceeds to do something which is not only monumentally stupid, but also endangers a whole lot of innocent bystanders.
Mind you, the author had realised that the kid needed more than just an ordinary lapse of all higher brain functions in order to make this seem likely. So he had him ingest a drug which had this stupidity as a side-effect, which kicked in during the 30-second span between him making an enemy, and him deciding to trust this enemy.
Dearie dearie me.
There are better ways of doing this, you know. He can be young and reckless, and make his enemy because of this recklessness, and do stupid and thoughtless things out of recklessness. (This could also have the side-effect of making your antagonist seem like a real person. ) I could relate to that, yes. Supposedly clever people being dimwitted because the plot demanded it, no.
I see the same, occasionally, when critting. There is a well-written piece of fiction, and suddenly, the author has someone - usually a minor character - acting very strangely, and in self-contradictory ways, simply because it seems like the easiest way to get on with the plot.
This is treating people badly.
Even bit players are the protagonists of their own stories. Let them have their own reasons for doing what they do, and let them stay true to their characters. If this means it takes you two extra paragraphs to get there, so be it. If you need to introduce another bit player to make the scene work, so be it. But whatever you do, don’t let your characters act out of character. Credibility is destroyed in a second.