In her 1987 work Unheard Melodies, Claudia Gorbman was influential in establishing the terms "diegetic" and "non-diegetic" (derived from narrative theory) for use in academic film music studies.
An early example was Jack Beaver's 'Portrait of Isla' from the score for the 1940 Edgar Wallace film The Case of the Frightened Lady.
Here, the piano is actually played by lead actor Marius Goring (an accomplished pianist) as Lord Lebanon.
If it is synchronized with the action, as in the "Good Morning" dance sequence from Singin' in the Rain, it is said to be Mickey Mousing.
[6] For instance, if a character in the film is playing a piano, or turns on a CD player, the resulting sound is diegetic.
Music can also becomes diegetic with the assistance of audio engineering techniques, having its reverberation undergo change to match the room's characteristics and indicate a spatial location from the surround speakers.
[13] Examples of diegetic music in opera go right back to its beginnings - for instance the central wedding serenade in Monteverdi's L'Orfeo.
is non-diegetic, since the musical material is external to the narrative, it being a conversation that would in a naturalistic setting take place as simple speech.
A solo piano (played onscreen) accompanies her, and the film's offscreen orchestra (presumably not heard by the characters) sneaks in for the second verse of the song.
[19] In the television series Buffy the Vampire Slayer, the episode entitled "Once More, with Feeling" toys with the distinction between diegetic and non-diegetic musical numbers.
It becomes clear that the characters are all too aware of their musical interludes, and that determining the supernatural causes of the singing is the focus of the episode's story.
On the same show, the episode entitled "The Body" was presented without any non-diegetic music at all, in order to convey the reality of the theme of death within the family that it portrays.