Below is a short article by Joseph Carl Breil on making special scores for films through adaptation of pre-existing music. This is from a three volume collection of short essays on various aspects of the film industry, and it was published in 1922. I have included a link to the original source, which is available through the Internet Archive.
MAKING THE MUSICAL ADAPTATION
By JOSEPH CARL BREIL
Scorist for "The Birth of a Nation."
Editor's Note [contained in the volume in which this piece appeared]: Joseph Carl Breil holds a unique place in the motion picture world. To his keen foresight may be attributed the present-day musical adaptation for motion picture productions. It was he who first saw the need for such adaptations, and his score of D. W. Griffith's masterpiece, "The Birth of a Nation," not only stamped him as the pioneer in this work but also immediately marked him as a master in the art of making the musical adaptation for the motion picture production. He has had the distinction of having a grand opera produced and has written scores for many hundreds of the finest motion picture productions.
WHAT music has done for motion pictures in developing their entertainment value and in lifting them to a plane never dreamed of by the pioneers of the industry is a matter of general understanding. To quote Mr. Arthur James, Editor of Moving Picture World, "Music took pictures by the hand and led them to greatness, and today pictures without music are not even considered for public entertainment."
Those sentiments, coming from so great an authority as Mr. James, speak for themselves. Since music has done so much for the motion picture in the past and up to the present time, it is but reasonable to presume that music will do even more for the pictures in the future.
So, there is a place for the musician in the movies. He has the opportunity to win everlasting fame by linking his name with the name of some great producer, director or star, by writing the score that will interpret the theme and moods of the play.
It is no easy matter to score a motion picture. And the musician who is ambitious to do this type of work must have a comprehensive knowledge of the music that has been written, and a high degree of appreciation of the values necessary, for the scorer must fit his harmonies to the elusive emotions of the players on the screen.
The scorer must be able to work spontaneously. He must register his thoughts on paper simultaneously with the
 pre-viewing of the picture. These are cardinal requisites of the motion picture scorer.
As soon as the producer has his picture in its finished form, he calls in the scorer, who must be able to take notes as he watches the drama. These notes indicate just what music he thinks will be most appropriate and most impressive when accompanying the various scenes.
Of course, the scoring is not a haphazard fitting together of various melodies that will interpret the different parts of the picture. There must be a general theme that will be in keeping with the theme of the picture. Selecting the general theme is in itself a difficult task, and a task that requires revising.
After the theme has been decided upon and the various pieces of music incorporated, the score is played by an orchestra for the producer who watches another preview of the picture. The producer listens and criticises, and accepts or rejects the score.
It would be useless for me to tell the lover of the photodrama how much music means to silent drama. Music is so much subtler than speech that it conveys to the audience the very finest shades of dramatic meaning. Music speaks to the audience without detracting from the action of the story, and without the interruption caused by explanations. It is a known fact that the layman feels rather than understands music. Therefore, in accompanying the silent drama, music makes a direct appeal to the emotions. It is easier for the actor to gain sympathy and understanding when music accompanies the picture.
The scorer finds a rich field in opera music. Certain compositions accompany certain dramatic situations on the operatic stage. These situations are of course to be found in the moving picture story. For instance, in the picture "Kismet," Otis Skinner as Haaj is about to stab the merchant. For that scene, Mr. Carl Edouardo, Director of Music at the Strand Theatre in New York, and scorer of all the splendid pictures screened at that theatre, selected the music which accompanies the scene in "Salome," where John the Baptist is murdered. Naturally, it interprets the scene in "Kismet."
 When the reels are projected in continuity during the musician's preview of a picture, a stop watch is used. The picture must be perfectly timed. The symbolic music is selected and the various compositions are so arranged that they will coincide with the running time of each scene as recorded by the stop watch.
When one considers the infinite variety of emotions, themes, atmosphere, and the many sudden flash-backs that constitute this motion picture drama, it is easy to understand how difficult a task scoring a motion picture really is.
To select from the hundreds of thousands of pieces that have been written that one selection which is in perfect harmony with the scene that is being enacted, is the scorer's task. Not only knowledge is essential; time and patience are just as important.
But it is fascinating work. Persons qualified by musical knowledge and musical training will find it attractive. There is ample opportunity to exercise one's creative instincts, and the musician will find that he is amply rewarded for such creative accomplishment. It is a pleasant and dignified position, and the scorer of the great motion picture classics is finding his name flashed on the screen along with the name of other artists who have helped to make the picture.
To brother musicians who are looking for fresh fields where their efforts will be recognized and acclaimed, I advise giving this work a trial. If you have the qualifications—if you can prove your ability by some clever piece of scoring, you will find that a royal welcome awaits you in the motion picture field.
Source: Photoplay Research Society. Opportunities in the Motion Picture Industry—and How to Qualify for Positions in Its Many Branches. Vol. 2. 3 vols. Los Angeles: Photo Research Society, 1922, pp. 85-87.
Images of Breil from Library of Congress, Prints and Photographs division. First image; second image