
Diegetic and Non-Diegetic Sound
The crossover
There is a record of films we know that use purely non-diegetic music to construct a film, such as The Artist which was known to receive critical acclaim. Yet whilst come films incorporate diegetic and non-diegetic sounds separately it's vital for a film's success to use both and in many cases, films and TV shows have been known to make this combination more elaborate. Take the TV series, Glee for example. It's common we see that when the characters suddenly go into dance and song (diegetic) the song then creates a sound bridge for other shots of characters and events in other places (non-diegetic music) before it is then brought back to the original place (diegetic).
Another complex crossover features in The Lord of the Rings: Return of the King. In this third and final film we are taken to a scene where Lord Denethor tells Pippin to sing a song. As Pippin begins it is only the characters in the hall that can hear him (diegetic). However when the scene is cut with shots showing the battle between a legion of orks and Faramir's army, the music is now non-diegetic as it forms part of the movie's score. When Pippin nears the end, the song becomes once more, a diegetic song and it is no longer non-diegetic.
Glee - Everytime (Full Performance) (Official Music Video)
LOTR Return of the King: Pippin's song - whole scene