Diegesis is a technical term literature can do without. Mimesis too, but we should keep that one simmering on the back of the stove for historical reasons.
“Diegesis” is related to one of my favorite topics, namely Show, Don’t Tell and the reservations I entertain with regard to the latter in extend to the former. There’s a rather lengthy article on diegesis on Wikipedia, but most of it is related to movies, theater, and roleplaying games. So let’s start with The Glossary of Literary Terms.
Nothing! But of course Abrams has a substantial entry for “mimesis”, of which “diegesis” is said to be antagonistic to. But let’s retreat and regroup and check out Wikipedia’s entry for diegesis:
Diegesis (Greek διήγησις) and mimesis (Greek μίμησις) have been contrasted since Plato’s and Aristotle’s times. Mimesis shows rather than tells, by means of action that is enacted. Diegesis, however, is the telling of the story by a narrator. The narrator may speak as a particular character or may be the invisible narrator or even the all-knowing narrator who speaks from above in the form of commenting on the action or the characters.
For both Plato and Aristotle, “diegesis” reports rather than represent, narrates rather than embodies, tells rather than shows. As with other conceptual opinions Plato and Aristotle entertained about poetry, I think “diegesis” should also be scrapped, once and for all, for the field of literary texts. What can’t be scrapped that easily is the term “mimesis”—but not because it makes more sense than “diegesis.” Mimesis as a concept has been exploded from formalism onward, and all Graff’s horses and all Alter’s men can’t put mimesis together again. (But of course we can discuss that in the comments.) The difference is that mimesis—as a concept and as a term—is much more ingrained in Western culture than diegesis, and we can’t just throw mimesis out if we want to speak intelligibly about art or the history of literature.
Even for Greek tragedy the term “diegesis” doesn’t make much sense, really. Greek tragedies had much less “show” elements than “tell” elements compared to contemporary theater, granted, but it’s not like they didn’t have “show” elements at all. Besides the parts of the chorus, much is related by dialog, and for dialog, talking about diegesis vs. mimesis doesn’t make much sense either if you think about it. Plus, you have to add parabasis and quite a few other very odd things, and the best that could be said about mimesis and diegesis in this context would be that they mark two fuzzily defined positions connected by a slider bar. Yet, among literature’s most defining characteristics is its ability to engage the reader directly with the characters’ thoughts, what’s going on in their minds—and there you can’t show, you just have to tell, and that’s why movies and other media are very clumsy at it indeed. Unlike literature, movies and other media don’t have the power to directly “tell” what’s going on in the mind of a character; they either have to fall back to what are essentially literary techniques, like VO, for example, or they have to go to great lengths with “showing” techniques that still fall considerably short, and are much more ambiguous and fuzzy, compared to what literature can do, no matter how advanced and sophisticated these techniques might be. And, here and there, differentiating between diegesis and mimesis isn’t helpful at all.