Lots of the magic behind great films can come from a carefully-selected soundtrack or original score.
FOX
Although subtle, they can be an integral part to the storyline by communicating emotions and establishing a ~mood~ for the audience.
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Just think back to that memorable shower scene in Psycho, which was made even more terrifying with the accompanying screeching violin.

Universal Pictures
Or what about the magical sounds of "Hedwig's Theme", which helped bring the world of Harry Potter and the Philosopher's Stone to life.

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And there's no forgetting the goosebumps-inducing melody of Inception.

Warner Bros. Pictures
As a personal, little favourite, Her also deserves a honourable mention because that soundtrack keeps me up at night.

Warner Bros.