Diegetic: sound and image exist in the story's world—radio music, dialogue. Non-diegetic: score, voice-over, titles—only for the audience.
Diegetic / Non-Diegetic
On set, you quickly notice: there's sound and image that the characters themselves perceive — and those that only we see and hear. Understanding this distinction is fundamental to every production decision, from the screenplay to the mix.
We call everything that exists within the narrative world itself diegetic. The TV is on in the living room — the character turns it on, hears it. The orchestra is playing in the concert hall — the camera is in the audience, the music is real in that moment. A car accident, rain on the window, the neighbor rings the doorbell. If you're shooting as a DoP and the sound mixer recorded the classical music from speakers because it's playing in the scene — that's diegetic. The characters can later say: "Did you hear that?" That's the rule.
Non-diegetic, on the other hand: the soundtrack under the opening scene. A voice-over from the main character revealing their inner thoughts while they silently gaze out the window. Titles and credits. Graphic effects, transitions. This directly addresses you as the viewer — the characters don't perceive it. You often work on this later as an editor or mixer: you add the music in editing and mixing, not on set. There's no orchestra in the room, no speaker.
The gray area is the interesting place. A character hears music through headphones — diegetic or non-diegetic? It depends: do we see the headphones, do we see them react, can others address them — diegetic. Are they lying motionless and the music only floats for us while their facial expression remains blank — precarious. Sometimes you consciously blend them: the music starts diegetically (radio in the car) but extends into non-diegetic areas, becoming a storytelling tool.
For post-production, this means: separating the mix into layers. Diegetic sounds are location sound, production audio — these need to be clean, need to be separable from the rest. Non-diegetic elements are your creative interventions afterward. Camera placement, editing rhythm, the music itself — all of this speaks to us, not to the characters. If you mix or blur these without realizing it, the film becomes confusing. Audiences feel it instinctively.