Diegetic sound is any sound which has been presented within the films world's. One reason why film companies use this type of sound in their film to help hook the audience into the film and involving them into the story. Diegetic sound includes dialogue and footsteps. Changing from diegetic to non-diegetic also draws the audience into the story more, however you can also pull the audience away from a scene of a films by switching between the two.
Non-diegetic sound is one that characters on the film set will not be able to hear although the audience will be able to hear it such as a soundtrack. This can help emotions flow from the audience, for example if you put a slow, soft, generally sad song in the background of an emotional film will set the scene well. Non-diegetic sound can also help the use of some special effects such as explosions in an action film or heavy breathing in a thriller.
