Briefly explained, diegesis has to do with the telling of stories, as opposed to mimesis which has to do with showing. Please consult the module on Narrative for more in-depth information about telling and showing.
Diegesis says something about the level of narration. A frame story in which a character-narrator arrives at a house is one diegetic level. A story being told to this person by an old housekeeper there takes place on a different, lower, diegetic level. A story told to the old housekeeper and included by her in her narrative takes place on an even deeper level of diegesis, and so on. Narrative voices are fairly rare in stage plays (cf. choruses), and voice-over is considered something of a clichè in films, but both are at least possibilities, unlike in novels, where narrative is pretty much a necessity. Diegesis is much more commonly talked about in conjunction with prose, because drama contains a strong element of showing (i.e. mimesis), but the distinctions are usable in drama as well.
Extradiegetic information comes from ‘outside’ the narrative and affects it in various ways. In drama it usually takes the shape of music, lighting or recorded voices. Music that the characters can hear is diegetic (for example, a song playing on a car stereo); music that only the audience can hear is extradiegetic (for example the orchestral score to a movie).
Stage directions exist on a level where the characters are not aware of them, even if the actors playing them are. So, stage directions too are extradiegetic in a manner of speaking. The theatre, however, is to a large extent a mimetic form of art; stage directions are important because they show us what stage persons are doing.
It is a useful notion to keep in mind as we move on to look more in-depth at examples of how stage directions work in drama. We shall begin by looking at a play endowed with a wealth of stage directions.
Next: Many stage directions