Understand the idea
Diegetic sound belongs to the story world; nondiegetic sound, such as a musical score, need not be heard by its characters. A sound can also be offscreen while still belonging to that world. These are different distinctions. Sound can direct attention, connect scenes, disclose unseen action, or create a gap between a character's knowledge and the viewer's. Analyze both source and function rather than equating all music with background decoration. [S21]
Worked example
In a fictional corridor scene, add footsteps outside the frame. Next add an ominous score. The first suggests a source characters could hear; the second may guide only viewers. Finally, remove the score and describe what changes without assuming silence always means safety.
Where the idea stops
Voice-over and music can have complex or shifting story-world status. Identify what the scene establishes rather than assigning every sound a rigid category. [S21]
Connect it
Psychology and literature: expectation, perspective, and withheld information.
Go deeper
Can a sound be both offscreen and diegetic?