Diegetic Sound

Diegetic Sound is any sound which originates from within the film, meaning the sound can be heard by the actors. This can be dialogue, sounds made by objects in the film, music/instruments played by the characters and ambient sound.

If you're watching a movie scene where a man is sat down at his computer drinking a cup of coffee, you're more than likely to hear the diegetic sound of his fingers typing, the mouse clicking, the placing down of his cup etc. What's not immediate is that most of this sound is likely to have been recorded separate of the shots, put into editing, dubbed and thus it can be classified as a diegetic sound effect. Although it's part of the scene and he hears all of this, it is used in post-production and can be known as Diegetic DIY.

The focus of a film is to build narrative and it's just as important that the music is right to create an effective film. Diegetic sound is one of the most important uses of sound, used in order to produce an authentic movie. Without it, it can't build character or sustain realism. For example, here is a clip from The Mortal Instruments movie, formed entirely from diegetic sounds. In it we hear Jace playing the piano with enough force that Clary is left to remark the strong emotions he must be feeling. Jace simply provides a witty response. In the clip we hear: the piano, dialogue, the scrape of fabric and the barely audible sound of the music sheets being shuffled. For one, the sounds bring this scene to life because there is enough sound to make it realistic. For another, narrative is built because there is a lot to infer from the scene e.g. we know about Jace's inability to detach emotion from his music, he is witty and from the easy tone in their voices there is evidently some relationship between the two.