Korean novelist Yung-ha Kim gave a TED talk entitled "Be an Artist Right Now."
He pointed out that there are many reasons that we, as adults, resist the impulse we may feel to tell stories. One is that it seems daunting to write something as long as a novel or screenplay. In answer to that, he quoted Roland Barthes on how Flaubert wrote: "Flaubert did not write a novel. He merely connected one sentence after another. The eros between sentences, that is the essence of Flaubert's novels."
Yung-ha Kim said, "That's right--a novel, basically, is writing one sentence, then, without violating the scope of the first one, writing the next sentence. And you continue to make connections."
Maybe that makes it sound too simple, but I've found that often when a book or a screenplay doeasn't work it's because the story takes a sudden turn in the direction of what the writer thought might be an interesting scene but that in some way betrays what went before.
If you're hesitating about writing, start now with one page. Tomorrow write another one. If at some point you feel ready to work out the rest of the story, go for it. Otherwise keep following one page with another and have faith.
(The great writers like Dickens and Austen and Twain had advice to help writers. I've collected it all and added tips on how to apply that advice to your own novel or script. The book is called "Your Creative Writing Masterclass" and you can buy it right now at Amazon or your other favorite bookseller.)