Tripped over this while searching through emails today. A friend of mine—already very smart and a good writer—was interested in writing a screenplay. From my Out Box, January 24, 2007:
Sally Hogshead was a speaker at the Screenwriting Expo this year and she mentioned the way to success in your career is getting clear with what you can do that nobody else can, and then hammer at it persistently. I like both of your stories, a lot, and think they have a good chance of becoming excellent dramas with their uneasy mix of moral ambiguity. These kinds of stories set your work apart from everyone else.
Here's a good first step: break down the first story into its necessary scenes. Identify exactly where the major story points are: Setup, Turning Point #1, Development, Turning Point #2, Climax, and Resolution. (It sounds like you've done this already.) Then flesh out the number of scenes needed to establish each section, adding in detail and dialogue where needed. You'll have a good 4 to 5 page treatment when finished... I used that kind of outline/treatment as a rough guide when I wrote Arousal and wrote the first draft in about a month.
If you're still in love with your story after writing the treatment, great! If not, then keep reworking it until you are. Next step is writing the script, which needs a whole other bag of tricks that I can help you with when the time comes.
But a few disclaimers would be well placed here:
The most important thing to remember is that screenplays are a visual medium... not a novel. Screenplays, then, are a blueprint for a final product and until they're being produced, they are effectively a business card for the project. They need to read very well to get others excited about helping produce. Imagine that a screenplay reader is in a hurry for work and can only read a page... but your script is so good that they read 2, then 3, then 5, then 10... until they realize they're on page 60 and are so late for work that they might as well finish it. Writing with long novel-like paragraphs is not, in my opinion, the best way to write a screenplay. 3–4 line paragraphs at the max. Few long speeches. Lots of white space on the page. Leave the director to come up with the fancy visuals later, even if that director is you.
Another common confusion—the objective with scripts is not to make a film from that script, but to sell the script to make the film and THEN write the script that you want to film. Scripts change a lot from the time they're bought to the time they get produced, so first write a script that gets people excited about handing you money, then swap it with the exact words you want to film.
Finally, scripts aren't stage plays. Filmmaking involves collaboration and the established norm is that everyone wants to add their 2 cents, sometimes holding back money unless they get some creative control. If you can find an executive producer (the money guy) who gives you complete creative control, then you'll be very very lucky. It took me years to accept this horrid truth. Still, many talented people out there can actually improve my writing, so it's not always a bad thing.