For example, most Star Wars trailers after A New Hope used portions of John Williams' score for that film, whereas the original trailer for A New Hope used stock music! Once the score is completed, the studio might release a new batch of trailers using the film's own music. The first film of a franchise is most likely to use this trope, whereas trailers for future films tend to reuse music from the previous ones. Slap in some music from another film, stock music, or even a pop song, that evokes the general mood they're going for, that's what. This means that there will be enough finished scenes to make a trailer, but no soundtrack yet.
When releasing a film to the public, the film's score is usually the very last thing to be completed. Danny Elfman, on the music written for the trailer of Mars Attacks!
