Hitchcock said in Psycho, after the shower scene, the audience did all the work. People scared themselves.
The master of suspense had already created a world that was more terrifying than viewers had ever seen, where the apparent female lead was dead, the stolen money didn’t matter, the world was cut off from help and normal rules, and the last major character(s) standing made no sense in terms of anything they’d seen before.
Classic horror movies often skipped special effects and worked to engage the audience’s imagination. The monster was never shown in the film or only shown very late in the story. But no one had ever created a monster as spooky and dependent on audience expectations as Norman Bates’s mother.
So one powerful way to add credibility to a story is to brilliantly set up the story and then dramatically reverse expectations. Pull the rug out from under them, and doubt loses its footing.
More often another technique based on emotion is put to work to make a story believable. The author uses a sympathetic character’s interpretation of incidents. By creating a protagonist people deeply identify with, writers can support that character’s selection of information, how it is ordered in its presentation, and the contexts for each bit to feelings build confidence in the legitimacy of what is observed, even though it might have logical inconsistencies. Once a character gains trust, his or her perspective is more easily accepted. Essentially, this a subtle (and presumably benign) form of spinning.
There’s one way any character (including the villain) can convince an audience (or at least get them to suspend disbelief). A monologue that is self-revealing can create an unquestioned feeling in people viewing a film or stage presentation. The requirements are that the monologue must be aimed at a specific character who is part of the story (and to whom a disclosure would make sense), the monologist must have a powerful and clear motivation for sharing something that matters deeply, and there should be an indication that a sacrifice is being made. For a monologue to be vital, it needs to be presented at a price.
Many times I’ve seen important truths included in a story and discovered that other people seem to miss them. I think this is not surprising when information is presented within the context of humor (and sneaks in under defenses) or where the story works at multiple levels (as often happens with the best children’s stories). But sometimes what the writer presents gets missed because of distractions or the way it is framed. In general, revelations need to be featured in some way. Get the audience’s attention first, then share the truth.
Respect is another important consideration. No audience wants to be talked down to or held in contempt. Unless the audience believes the writer sees them as an equal, those hard-earned treasures being offered will be rejected. We listen to people who have our best interests at heart, who care about us.