Writing a filmscript is a completely different kettle of fish from writing a book. Look at it this way. A book can be anything from 300 pages upwards, crammed with single line spaced text, describing all sorts of events and what the various characters are thinking, planning, and experiencing. A filmscript should be about 100 pages long, spaced out with character names before each piece of dialogue (which only stretches across half the page), and you can never write in what characters think - you have to show everything. You are writing for a visual medium, not the written page. The filmscript mantra is SHOW, NOT TELL.
So the mission, should you choose to accept it, is to force 300+ full pages down to 100 half empty pages, but show everything explicitly in the process.
Quite a challenge.