I often find myself writing half of a screenplay, and then throwing it on the scrapheap because my characters have totally lost their direction. How do I build on my characters to help me, by taking the story in their own direction?
In your case, I would urge you to really figure out the end of your story before you begin writing. Otherwise, it’s very easy to keep writing scene after scene and end up with interesting characters in a mess of a story.
One trap that many beginning writers encounter – especially those who’ve read some of the more notorious screenwriting books – is taking the truism "character-driven story" too literally. Yes, the most successful and engaging movies are those where the characters seem to be in control of their own destiny, where every turn of the plot seems to derive from an element of their personality.
But it’s naïve to think that all a writer has to do is come up with amazing characters and watch them go to work. The truth is, great characters are useless unless we see them doing interesting things – and coming up with those things is the screenwriter’s job. Don’t start writing until you know both who your characters are and what they’ll be doing.