Silent movies need music, and there are all kinds of ways to provide it. Sometimes there's a live musician, or, on special occasions, a live orchestra. Sometimes there's a pre-recorded score, with a full orchestra, a small band, an organ, a piano, whatever. Sometimes the score is totally original music; sometimes it's based on familiar music from the public domain. The silent-movie scores I've heard have one thing, and only one thing in common: they suck.
Okay, "suck" is too strong. I just said it to get attention. (I do so need attention.) But honestly, I've never been satisfied with a new score for a silent movie. Even the scores that are professionally done, well-played and don't sound anachronistic -- there are way too many scores that try to provide a postmodern sound for what is the cinema equivalent of the pre-modern era -- don't seem to add anything to the movie, and don't really seem to match the onscreen action. I could never quite figure out why I thought this, or what I thought a good silent movie score should be, until I saw a non-silent movie: Ernst Lubitsch's Trouble in Paradise. This movie -- one of the greatest film comedies ever made -- is a talkie, made in 1932. But unlike most talkies of the era, which used no background music at all on the mistaken belief that if you have sound you don't need music (some of the early sound horror movies, like Frankenstein and Dr. Jekyll and Mr. Hyde, are severely weakened by their lack of music), Trouble in Paradise makes extensive use of background music, composed by Franke Harling . In an effort to give the movie some of the fluidity that his best silent films had possessed, Lubitsch did many shots and even some entire scenes without any sound, and few if any sound effects; instead he used music to take the place of sound, just as it does in a silent film. Basically, then, the silent scenes in Trouble in Paradise demonstrate what Lubitsch expected music to do in otherwise silent scenes; and by watching and listening to these scenes, we can get an idea of what a great silent-film director would want a composer to do.
In the opening scene of Trouble in Paradise, the first time we hear music on the soundtrack is when Edward Everett Horton is trying to stand up (after having been knocked unconscious). The music isn't melodic; it's just a series of halting repeated phrases that mimic the action: Horton is slowly trying to get up. As he loses his balance and falls to the ground, a chord on the soundrack mimics that action as well.
That's the way it goes through the whole movie: in the scenes with no sound, composer Harling doesn't do what we would now expect a composer to do, which is to set the overall mood of the scene or enhance its emotional impact. What he does is follow the action: the music imitates movements, gestures, even footsteps. When Herbert Marshall runs up and down the stairs (actually, a stunt double is used for these scenes, since Marshall had lost a leg in World War I), there are no sound effects of feet hitting the floor; instead there's a rising musical figure for running up, and a descending figure for running down. In other words, it's the kind of music known as "Mickey-Mousing." You can see why people who compose scores for silent movies don't do this kind of thing, since directly following the action is considered gauche. And it is gauche -- in a scene that already has sound effects to go with those actions. But in a scene without sound effects, "Mickey-Mousing" works: it adds solidity to the actions, adds rhythm to the scene. That's what Lubitsch was looking for in silent scenes, and -- I'm going to assume -- in his silent movies: music that could connect actions to sounds, in the absence of any other sound to do that job.
Now, if you have the Criterion DVD of Trouble in Paradise -- and you should -- compare the new piano score that is used for the Lubitsch silent film included as a bonus, The Merry Jail. It's not a bad score; it's professionally put together and all that. But aside from the fact that it uses the wrong tunes (the film is based on Johann Strauss's Die Fledermaus, yet the score doesn't use any of Strauss's music; presumably Lubitsch would have expected Die Fledermaus tunes to be played to go with the movie, just as he would have expected Sigmund Romberg's music to be used for his silent version of The Student Prince), it's mostly just a series of melodies, a lot of mood music that sets the general tone of the scene; it almost never follows the action or mimics the characters' movements in that direct, Trouble in Paradise way. It just seeks to follow mood, rather than action; it's "underscore" music. The problem is, in a silent film there's nothing to underscore; the music is the only sound you hear, and it has to do the work that is done by dialogue and sound effects in a talkie.
Without that kind of action-specific music, the images just kind of float there onscreen, because there's nothing to solidify them. In this way, good silent movie music is a lot like traditional cartoon music. Carl Stalling, the composer for Looney Tunes (and, before that, for Disney), started as an organist for silent movies, and he carried that experience over to the vocabulary of cartoon music. Looney Tunes cartoons have sound effects for important actions, like explosions and falling anvils and such. But when it came to less spectacular actions, like walking, sound effects wizard Treg Brown (who worked closely with Stalling) would not put in sound effects for footsteps; instead Stalling's score would follow the characters' footsteps. Before the cartoons went into production, Stalling also set a rhythm for each scene, which the animators could animate to. This is, again, the kind of thing that you need in a silent movie: the music needs to set a strong rhythmic foundation for the scene, and where there are no sound effects, the music must in essence become a "sound effect" in itself, providing musical equivalents of physical actions.
So I say to anyone composing a silent-movie score -- and after all, who isn't? -- forget everything you know about good movie music; those rules are for talkies, movies where the rhythm is carried by sound effects and where you hear words when characters talk. Silent movies are different; your music has to do the work that is normally divided in three (between music, sound effects, and dialogue). Don't just play a tune or set a mood, because all that creates is a feeling that a bunch of people are walking too fast while some tinkly music appears out of nowhere. In a silent movie, the music must be tied to the action. If you're scoring, say, The General, it's not enough to play "Dixie" on the soundtrack; you need to follow Buster Keaton's walk, create music that suggests the roar of the train; when a puff of smoke appears, the music should play a chord to suggest that. In other words, don't be afraid of Mickey-Mousing. Because that's exactly what a silent movie needs.