One main question I have in film composing (as a beginner) is how to change a simple theme into many minutes of music? Let's say there's a scene where someone's walking outside in the moonlight and I already have a melodical theme for this guy. The scene has to reflect this guy's deepest feelings and trouble choosing the path to go in his life.
This is purely fictive assignment, but.. how do I start? How do I create music that is purely for background? Where could I learn about these techniques how to change a 10 second theme into a many-minute-piece?
I know know something about orchestration, but I'm missing the tools for creating this kind of piece. I can write a song with A, prechorus and B but I don't know how to start on a piece that has none of these.
Please, give me something to start with. How do you do accomplish this?