I personally think in many cases the difference between a script I love and a script I’ll pass on is the emotional density of the project. When reading, do I feel deeply for the characters – does their journey make me feel? Do I laugh and cry with them or for them? Do I want to fight for them and want them to win? Do I hate the nemesis and want him to not succeed?
Pay careful attention to why you want to start writing a project – the seed. When you come up with the idea, do you have a series of scenes, images, character qualities and quirks that come to you? Take careful notes about anything that is related to the seed of your story, because this likely has very deep emotional roots for you. If you can capitalize on this passion and get it onto the page, the passion will likely translate to the reader.
In my last 2 scripts, I had many of these such seeds that got written out as I was in the process and reconceiving. In the process of trying to make the story tighter and better, I wrote out a lot of these core nuggets, thinking that I was improving the project. However after doing the rewrites, I felt like I had stripped out a part of the guts of the project. I later ended up going back to those original seed moments and writing them back in.
There is something about the how and why of a story’s genesis, something psychologically deep about the way in which a story begins to show itself to the writer. We are teaching ourselves about ourselves. Even if we don’t understand it at the time, it’s probably very intimately linked to why we would spend months, a year, or more writing this particular story.
Go back to the seed, the concept. You’ll find what you need there.