Most classical music is meant to be listened to. That is, it is meant to be performed in a Concert Hall while people devote their attention to listening to it - with no other distractions. Consequently it needs to be sufficiently engaging to fully absorb the listeners attention.
Film music, however, is meant to support the action of the film. It just provides a background and works with the action to create an overall effect. Unlike Classical Music which can stand alone, film music often turns out to be quite unsatisfying once it is divorced from the images it was meant for.
Classical music usually has an internal structure. It can often consist of four separate movements with each of these movements being constructed according to a specific thematic pattern such as "sonata form", "rondo form", "theme and variations." This creates a musical narrative that the listener can follow.
Film music has no narrative of its own. It has to fit around the visual structure of the film and generally consists of a number of short "cues." Although these cues sometimes relate back to a common "main theme" or a small set of leitmotifs their aim is to quickly and unobtrusively suggest a mood to complement the visual image. The narrative is provided by the visual/textual aspects of the accompanying film and is not in the music at all.
The most successful classical music has always been that which is looking forward. Beethoven, Mahler, Schoenberg, Cage, etc wrote music that was cutting edge for its time and often disliked by audiences when it was premiered. Contemporary classical music is the most challenging and experimental music there is and often unpopular with mainstream audiences.
Orchestral film music on the other hand is usually looking backward. Composers like John Williams, Danny Elfmann an Howard Shore write music that superficially sounds like classical music did 100 years ago (Williams owes a lot to Wagner, Shore to Vaughan Williams, Elfmann to Stravinsky). This is the kind of music that the mainstream has had a long time to get used to and generally likes.
The difference is most apparent when they hold live concerts of Film Music played by a symphony orchestra in a concert hall. The audience consists of people whom like films and MOR - not classical music enthusiasts.
Some film music has been written with the intention of being re-worked into a fully developed classical piece. Vaughan Williams music for the film "Scott of the Antarctic" provided the basis of his Seventh Symphony. Erich Korngold regarded film scores as "operas without singing" and intended that his scores should survive without the film.
Such crossovers are rare though. Most of the CDs that are issued as tie-ins to films (e.g. Jurassic Park and Lord of the Rings) expose the music as being rather light on original ideas and rarely repay further listening. I am sure people will be listening to this music 100 years from now - but only if they are watching the film at the same time!
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