The music sounds so simple, far too few instruments and rhythm
You often hear this sentence when listening to film music without a picture. That's true, but that's exactly what makes film music. "Less is more". The image is always in the foreground. The more dominant the film music is, the more expressive the film scene has to be.
Well-composed film music often involves a lot of planning, knowledge and a feeling for the joint effect of image and sound. An important stylistic device are leitmotifs that characterize places or people in a film. They are a kind of memory aid for the viewer and make the film more understandable and structured.
In the case of the characteristic melody, the theme or motif, once designed, recurs unchanged over and over again. This makes sense with fixed locations and places in the film that are returned to again and again. It's not so much about moods here, but one develops a musical map in the overall film. The following applies to characteristic melodies: "Less is more", short melodies or sound surfaces, few striking instruments, such as the bagpipes for Scotland or the accordion for France.
Themes for characters can evolve and vary throughout the film and in mood and character. The following applies here: Short, clear topics. These are taken up again and again during the course of the film. This ensures recognition by the public.