A few years ago, I attended a Q&A session featuring Paul Schraeder, and he offered a suggestion for a way to prepare yourself before diving into writing a script.
Pick a friend. Maybe two or three friends. Invite them out for a coffee, or to a bar. Somewhere you can talk without a lot of distraction. Let them know that if you can bend their ear for twenty or so minutes, the first round is on you.
Don’t pitch a thumbnail of the idea. Tell the story. Start from the beginning and work towards your end. Try not to paint in too broad of strokes. Get in to the nitty gritty of what you have. Again, tell the story.
Watch your listener. See where their attention shifts and at what points they ask questions. Do they need something clarified because they’re confused? Do they want to know more?
What you’re looking to see is if your story can keep the listener wondering what happens next. There’s a mildly sneaky test for this: Build to a cliffhanger, or some other dramatically tense point, then pause and excuse yourself to the restroom.
If they ask you what happens next when you return, you’re on the right track.