What is diegesis?
Diegesis is a term used in film music to discribe the way in which the elements of film relate to the narrative the film is telling. In film music, it is simple way of describing the function of music: if it occurs within the world of the characters (i.e. someone could hear it) it is diegetic, if it occurs in the world of the audience (but the characters cannot hear/see it) it is non-diegetic.
How can you use it?
It’s not a technique as such, rather a lens though which you can look at your music. Although the script will usually give hints as to the most obvious diegetic cues, you can shift this factor to make your work more interesting: rather than a standard piece of underscore, you could make it play from a gramophone or radio during the scene; or you could use on-stage musician (such as a string quartet in a restaurant) to accompany and set the tone of a scene.
Blurring the line
Of course, the most exciting use of this technique involves the blurring of the line between the diegetic and the non-diegetic. This is particularly easy to accomplish in films (such as Philip Glass playing the background of the Truman Show, or the entrace of the brass band in My Fair Lady) but it can also take place in the theatre. West Side Story has a famous scene in which a jukebox is turned on and plays a recording of the music to an earlier scene, this then segues into “live” underscoring played by the pit orchestra. However, we still believe that the music is ‘coming’ from the jukebox (a consequence of Schitzophonia) albiet with a richer and more visceral style. This effect, the transition from one frame of reference to another, enhances the performance and gives the scene much more impact. Ironically, blurring or transitioning across the real/non-real line can make the performance seem more real by assisting or refreshing the willing suspension of disbelief.
Conclusion
Although not a technique as such, diegesis is an important factor in the construction of theatrical artifice. It can be used to change the perspective on a scene and to freshen an otherwise stale or unorginal cue, or to draw attention to or distract from a musical or theatrical element. Next time you are struggling with a music cue, think about whether you can change its position in the narrative frame to make it more interesting. At the very least this will give you some creative inspiration, at most an entire new way of presenting your score and sounds.