Film scores are often based on short themes, and it helps if you've got some way of developing these themes and making them sometimes last 4 minutes and sometimes last 40 seconds. One ends up doing it subconsciously.
I prefer to see a rough cut of the film rather than read a script. I find it difficult to get the feeling of the atmosphere of a film from a script.
The greatest thing you can have when you compose a film score is a bit of time to think before you have to charge straight in and do it.