Shooting a movie musical can be as complicated as an action blockbuster dominated by green screens and CG visual effects. For the past few decades, directors, for the most part, have preferred to have their stars sing live on set as opposed to lip-syncing a pre-recorded track.
That often means an actor is singing their hearts out listening to the background music coming through an earpiece as their voices project acapella to a crew of often fifty or more people on set.
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