What Is the Difference Between Diegetic and Non-Diegetic Sound?

What is the difference between diegetic and non-diegetic sound? Can they ever be confused? Discuss with reference to at least three recent films.
For this essay I shall be highlighting the differences between the two terms; diegetic and non-diegetic sound. I shall also discuss whether or not the terms and their meanings could ever be confused. To help highlight my arguments within this essay I shall reference to films such as "The Italian Job", "Romeo and Juliet" and "Entrapment". The final section will hopefully round off the essay with a critical conclusion of the given question and the evidence presented within the essay.
Yet sound is perhaps the hardest of all techniques to study... Our primary information about
the layout of our surroundings comes from sight, and so in ordinary life sound is often simply a background for our visual attention.
(Bordwell.D. & Thompson.K., 1947, Film art : An introduction, Fifth Edition, New York; London : McGraw-Hill.)
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