I've been working on developing a screenplay since the end of 2006. I got a spark of inspiration while I was watching Metropolis of all things.
I formed a high-concept idea, and I had a setting but no story. In early-2007, I looked at a script I did for a Screenwriting class in 2002. For the earlier script (which was atrocious and I knew it even as I writing it), I had the opposite problem: a story but no setting. So I put the 2002 story into the 2006 setting, and started from there.
I took note of all the story points from my earlier script and discarded everything that unusable or just garbage. With what was left, the new setting meant several of those story points had to be changed and adapted.
Once I had my basic story points, I began to plot a new synopsis. First a one-page synopsis, then a six-page synopsis, then I kind of stopped. I decided to step away from the story and I wasn't satisfied with what I came up with, not believing that I really did justice to my idea.
In July 2008, I began talking to a friend at a pool party about my idea and before I knew it, I had a co-writer.
During the second half of 2008, we introduced new characters, fleshed out pre-existing characters, gave a better motivation to the characters, and started plotting out how scenes could work. I wrote a new six-page synopsis. I knew the first act had to be 25% of the story, the second act 50%, and the third act 25%, so when the first act started to become too long, I made the second and third acts proportionate. I moved some things from the end of the second act into the third. A lot of the second act was filler or taking up time (still!), so then I put all my focus into making the second act a middle instead of a muddle. When I was done with the long-synopsis, several changes later, it was 15 pages. Half the length of a treatment. It was February 2009 by this point.
I wanted to do a full treatment but my co-writer was anxious to start scripting, so that's what we did. My partner focused on the first act, while I did wrote the second. The second act is the hardest and really the second act is the heart of a story so that's why I wanted to write that. We'd get to the third act and develop it together. We started scripting in April 2009.
When we started scripting, we found that certain things worked better in synopsis form than in dialogue or scenes (part of why I wanted to do a treatment first), so we've had to rethink things. My co-writer is still finishing the first act (yes, I knew it's 17 months later), and I had to throw out the entire synopsis for the second half of the second act. After I rewrote the second half of the second half, I scripted it, and the entire second act turned out to be 63 pages.
I decided to take a break after that to wait for my co-writer to catch up, then after a few months decided to take another look at the second act and cut 20 pages out of it. I decided I couldn't add any more into what used to be the "second half" so decided to look at the "first half" and decided to shift a few scenes from the end of the first act into the second and expand them into their own sequence. Scenes my co-writer hadn't gotten to yet. I saw that we could've used a bit more set-up transitioning from the first act to the second and everything was too abrupt. My co-writer agreed, and that's where I am now: scripting the beginning of the second act and connecting it to the rest of what I have.