I really liked how the tempos reflect the same time warping ratios experienced by the characters in the different dream levels.
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Just for the game of it, all the music in the score is subdivisions and multiplications of the tempo of the Édith Piaf track. So I could slip into half-time; I could slip into a third of a time. Anything could go anywhere. At any moment I could drop into a different level of time.
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If you were to see this movie a second time, you realize the last note you hear in the movie is the first note in the movie. It’s a Möbius band. But the next thing you hear over the logos is actually telling a story. You realize that the elements that we’ve extracted from the Piaf song are the way you get from one dream level to the next. When the movie starts, some action has already happened.
Hans Zimmer Explains the Intersection Between Edith Piaf and the Inception Score