In the world of films and film music, dubbing is the process whereby a new soundtrack is added to an already completed film. This could mean adding dialogue, sound effects or music.
Dialogue might have to be dubbed in if the original recording was deemed unsuitable. Most (in)famously, David Prowse’s performance as Darth Vader was dubbed over by James Earl Jones (allegedly without Prowse’s knowledge).
Sound effects can be dubbed in to create a desired mood in a particular scene. Screwball comedies often did this by dubbing in slapstick sound effects to emphasize comedic falls or double takes.
Music, both diegetic and non-diegetic, might have to be dubbed in if the original recording was damaged in some way or the director becomes unsatisfied with the initial result. For example, say a movie originally features a certain diegetic song playing on the radio. If the director changes their mind and wants a different song, the editors would go back and dub in a new song, so that the audience would never be able to tell that a new song was added in.
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