Diegesis

Diegesis is the fictional world of a narrative work considered as a totality — everything that exists or happens inside it — as distinct from anything presented only to the audience.

The clearest application is sound: a diegetic sound has a source inside the story world, audible to the characters — dialogue, a radio in the room, footsteps — while a non-diegetic sound reaches only the audience, like a film score or narration. The same split runs through interface design: a diegetic health bar exists as an object a character could see, tattooed on a forearm or shown on an in-world screen, while a non-diegetic one is a heads-up overlay the player sees and the character does not, structurally identical to Subtitling that translates speech no character hears. The non-diegetic layer is not neutral about how the diegetic one is read: a score laid over an unchanging face tells an audience what the face means, which is the Kuleshov Effect working across the boundary rather than between two shots.

The boundary is not a fact about a medium, it's a choice, and where a work draws it changes what the fiction claims to contain. A game that puts its map and objective marker inside the world commits to justifying their presence there — Environmental Storytelling often does exactly this. Crossing the boundary on purpose, addressing the audience through a device characters cannot perceive, is a stronger move than a slip; a fourth-wall break works because it violates a held convention, not because breaking convention is inherently interesting.

Mismanaging the line, rather than deliberately crossing it, is one recurring source of Ludonarrative Dissonance: a system that only makes sense from outside the fiction — a respawn timer, a save file — sitting uncommented against a story that claims permanence and consequence. Whether that reads as a flaw or as accepted notation depends on whether the work asked the audience to take the layer as part of its claims at all, the same question Chekhov's Gun asks about any emphasised detail.