Diegetic & Non-Diegetic Sound...

Diegetic sound is sound which is sound which comes from a source within the scene and can be heard by both audience and characters, for example T.V playing, dialogue between characters, phone ringing, footsteps etc. Non-Diegetic sound is sound which has no source within the scene, the audience can hear it, but the characters cannot. Diegetic & Non-Diegetic sound can be used in many ways, some of which are shown in these two clips;

http://www.tcm.com/mediaroom/video/283936/L-A-Confidential-Movie-Clip-Open-Hush-Hush.html#

This is the opening of LA Confidential. Diegetic sound is used in the end of the clip, as it starts off with non-diegetic sound, the voice over and the music. The effect of this is to introduce the film, the plot, the era etc. It then turns into Diegetic sound as we hear the type writer sounds, and we see who is talking. This is an effective technique as it allows the audience to be introduced to a key character straight off, and are introduced to other key characters within the film opening.

The second film opening is similar to that of LA Confidential in that it starts off as non - diegetic sound, for example the creepy music playing while the key character moves up the stairs towards the victim, and keeps playing until the victim wakes up. We then hear waves splashing just after the opening credits finish, we are then shown where the characters are, in this case the beach, we then see where the wave sounds came from and the sound becomes diegetic. The effect of this is to create not only tension, but suspense. We don't know where the characters are in the beginning, but the audience can tell, from the music that something bad is going to happen. We are also introduced to key characters, through not only dialogue but who we also saw on the screen.