Audience holds two contradictory beliefs simultaneously — creates discomfort and emotional tension. Hitchcock exploited this better than anyone.
You know it from the set: The viewer is sitting there, simultaneously knowing two things that contradict each other – and that's exactly what makes them uncomfortable. This inner tension, this scratch in the head, that's cognitive dissonance. It arises when two beliefs, values, or pieces of information collide, and the brain tries to reconcile them. As a filmmaker, you consciously use this to create tension that doesn't come from action, but from a mental conflict.
Hitchcock was a master of this – he showed you the murderer in the first shot, thus giving you information. Then he shows you how this person lovingly treats their family. Your brain can't put together: Murderer + caring father = contradiction. You become uneasy. That was precisely the intention. This technique also works more subtly: a trustworthy character says something that contradicts their previous behavior – and suddenly you wonder who you can trust. The uncertainty itself becomes a weapon.
In practical editing or dramaturgical planning, you use cognitive dissonance through editing sequences, sound design, or dialogue timing. Show an image, play contradictory music to it, have a character contradict themselves – the methods are numerous. Important: It must remain unresolved, at least for a while. As soon as you resolve the dissonance (e.g., by making it clear: Aha, he wasn't really the murderer), the tension dissipates. This only works if the viewer remains in this uncomfortable state of suspension.
Distinguish this from mere confusion – cognitive dissonance has an emotional component. The viewer logically knows what they are seeing but feels something else. A drama can work with this, as can thrillers or psychological films. But dissonance can also work in comedy if you juxtapose expected behavior with real reactions. The art lies in using it sparingly – too much, and the viewer switches off; too little, and they don't even realize they're being manipulated.