Know what the job of music is in the movie and be prepared
Before you hire a composer, think about what your movie needs.
Watch movies in roughly the same genre/style as your own movie, listen to what the music does there and get a basic understanding of the musical stylistics of this genre. Also, know what the task of music in general is. It is NOT to help slow editing or bad acting. It can help there but it is not the task of music in the first place. Also, it is not the main task of music to simply comment.
The best job for music is to add another level to the visuals, communicate things that aren’t visible or obvious. There’s a neutral dialogue going on but you want a hint of tension because the dialogue deals about something which later becomes very threatening? That’s a perfect job for the music. Watch your movie from the perspective of where the music might be able to add another level.
Generally, music works very well to “glue” your story together. That function starts at rough scene changes that get smoother with a music cue and ends with large connections made by themes which can create a cohesiveness between dramatic moments. We will have a look at specific points in movies that are great for music later.
If you don’t listen to film music very often you should try to get your hands on as many film music of the style of your movie beforehand and listen thoroughly in order to develop a feeling for the music and its style. Also develop a sense of the amount of music. Certain movie styles have only very few music while others have music almost wall-to-wall. Get a feeling for the right amount of music.