Diegetic vs. Non-Diegetic Sound

Diegetic sound is implied in the world of the film, whereas non-diegetic sound is not present in the narrative universe, but is used to complement the visuals and give it an emotional effect. Another name for diegetic-sound is actual or literal sound, and some common examples are the dialogue of actors on screen, or the sounds from objects that are being shown. On the other hand, non-diegetic sound is best described as commentary or non-literal sound, as it takes the form of several instances that comment on the visuals, like a musical score or a narration. A good example of diegetic sound is in the movie "Braveheart" when the main character tries to convince an army to join his cause, and the sound is implied to be coming from his speech. When looking at films with non-diegetic sounds, an example would be the ending of "Road To Perdition" because when the little boy gets out of the car, a score from Thomas Newman plays that increases the emotional impact of the ending, although it's not implied to actually be in the world of the film.