The composer is the person who writes the score for the film, which can involve anything from a full-piece orchestra to a solo acoustic guitar. The composer usually works with the director in developing an artistic vision for what the music should sound like, although sometimes the composer is given free rein to write whatever he feels is appropriate. Some of the more famous film composers include John Williams, who wrote the score for "Star Wars" and "Schindler's List," and Bernard Herrmann, who composed the score for some of Alfred Hitchcock's most popular movies.
The music editor has essentially the same role as a film editor but works in the realm of audio instead. This person takes all the music, which includes both the score and the soundtrack, and makes sure that it fits as well as it can into the flow of the movie. This includes trimming certain parts of the score, adding fade-ins and fade-outs, and judging the overall volume of the music throughout the film. Of course, the work also has a visual component since an editor must also make sure that the music is always in sync with what's happening on the screen.
As with the job of music editor, a music supervisor does not technically have to be a musician. The work usually revolves around the soundtrack and working closely with the record industry in order to license certain songs for the film. Some music supervisors have a knack for picking the latest hip songs that can give a film an edge and an extra selling point. Others direct all the musical decisions, such as hiring editors and composers. In this sense, the music supervisor position can be the most important role in film music.
In film production, conducting is more of a specialty than other music-related work. The job simply involves coming into the recording studio and conducting an orchestra while the score is put on tape. For movies that do not have an orchestral score--for example, because the music is guitar-based or electronic--a conductor is not necessary. The qualifications for film conductor are the same as they are for any conductor and include being able to read sheet music and work with large ensembles of classical musicians. In particular, conductors should know how to get the best performance for a studio recording, as opposed to a live performance.