Diegesis is the opposite of “mimesis”, according to Plato. In diegesis the author speaks in their own voice, whereas in mimesis they do not (instead they try to create the illusion that it is someone else who speaks). In the 1960s, French structuralist scholars like Christian Metz and Gerard Genette adapted it to refer to the actual succession of events in a story-i.e. what happens which they opposed to the narration or telling of the story (effectively the new form of mimesis). This distinction found two particular uses in literary and film studies. First it allowed the separation of narrators who participate in the story which they are telling and those who do not think of Joseph Conrad’s Heart of Darkness (1900): Marlow participated in the action of the story he tells, so he is a diegetic narrator; but the person he tells the story to, who is in fact the one who reports to us as readers does not, therefore he is extra-diegetic, that is, outside of the story. Similarly, music in film can be either diegetic or extra-diegetic, part of the story being told, or an aural backdrop.
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