Backing Music for the rough cut

On the rough cut I compose the music to make the audience make the fell what I want them to. For example I made it scary music when it needed to be and I also made so it is sad music once the victim dies. I added in sound effects for the gun shots as we filmed with BB-guns which doesn't make any sound. I found this quite difficult as I could not always find what exactly what I wanted. I would have preferred to compose the music on a keyboard.