Gap between what's said and what's meant—or expectation versus visual reality. Only works when the audience catches the contradiction.
Irony thrives on the gap between surface and truth — and this gap must be visible, otherwise it doesn't work. In film, this operates differently than in theater or novels: here, the camera plays a role in whether the audience understands the trick. You can have a character deliver a line with complete seriousness, but if the visual composition or the music suggests something else, the rift is created. This requires precise craftsmanship.
The classic variant: a character says something that sounds superficially positive or naive, while the visual context (facial expression, editing, environment) reveals the opposite truth. Think of an agent explaining to a victim how much they will be helped — while in the background, the trap is already closing. The audience sees both levels simultaneously and understands the irony through this contradiction. Without this visual complementarity, it remains a mere lie or poor acting.
It becomes dangerous when the irony is too subtle: the audience misses it, and then the scene appears unintentionally comical or confusing. Or you overdo it — then it becomes cartoonish. The DoP and the editor must support the staging, not undermine it. Contrast in lighting, editing timing, sound design — everything must work together. A director who loves irony (Coen Brothers, Paul Thomas Anderson) knows: the camera holds a moment longer, the cut comes a frame later, the music contradicts the image.
Also, distinguish between dramatic irony (the audience knows more than the character) and situational irony (reality defies expectations). In film, dramatic irony often works better because we see images that lie to the character — and we see the truth. Situational irony requires stronger editing and монтаж work to remain understandable. Caution: Do not confuse irony with sarcasm. Sarcasm is an attitude, irony is the structure of contradiction. In an image, you need this structure, otherwise it's just provocation.