The film's storytelling perspective — who or what narrates. Can be invisible (classical narrator), embodied (voice-over), or ambiguous (unreliable narrator).
You notice on every set whether the story belongs to you or someone else. This is the narrative instance—the invisible hand that decides what you see, what you know, when you know it. On set, you rarely ask yourself this so explicitly. But in editing, when you add voice-over or decide whether a camera movement follows a character's gaze or is independent of it—then you are actively working on this instance.
Classically, this works invisibly. The camera shows you the world, no one speaks into your ear, and you forget that someone is even narrating. This is the most widespread narrative instance in feature films—the so-called objectified third person. You see what the director wants to show you, but the narrative instance remains hidden behind the apparatus. This works so well because it feels natural. As a viewer, you don't feel addressed, but rather immersed in a world.
As soon as a voice-over comes in—be it a character's stream of consciousness, the diary of a dead person, or an authorial narrator who comments—the narrative instance becomes embodied and thus questionable. This voice has a position, often an agenda. It doesn't quite trust you; it has to explain to you what you should see. This is dramatically powerful, but can also distance viewers if it's unclear why this narrative voice is necessary. In editing, you quickly notice whether voice-over carries the story or whether it only states what the image already shows—redundancy is the enemy of good narrative instances.
The unreliable narrative instance is the variant where you actively don't know whom you can trust. The camera follows the gaze of a character who is themselves confused. The voice-over deliberately lies to you. This makes the story unpredictable and forces the viewer to actively engage. As a DP or editor, you then have to decide very consciously: Do I stay with this character, even though they don't know what they're doing? Or is there a cut where the camera emancipates itself from them?
The choice of narrative instance determines the emotional proximity to the material. Subjective camera and first-person perspective make you the character. Objective, neutral observation makes you the viewer. And everything in between must be consciously designed.