This article by Scott Myers from Go Into the Story (a blog I highly recommend you look into if you are interested in screenwriting) offers some great insights into screenwriting by successful writers. I like the one below by Chris Sparling.
“No matter how big or high concept your story is, it only works if there’s a small, personal story at its core.”
So what’s the personal story you want to tell within the big story? What’s your story’s ESSENCE?