Diegesis

The world of the story itself, and everything that belongs inside it. Music the characters can hear is diegetic; the score behind them is not.

Diegesis draws the border between the story's world and the film's address to us: the radio the character switches on is inside; the strings that swell behind her are outside, audible only to the audience. The border is invisible and absolute, and every sound and title sits on one side of it.

Crossing it knowingly is a power move: the score that turns out to be a passing band, the narrator the characters suddenly hear. Each crossing reminds the audience there is a teller, which either deepens the spell or breaks it. The discipline is knowing which side each element serves, and crossing only on purpose.