Go pick up a copy of Getting Things Done. This isn’t a suggestion. It’s a requirement.
Your characters, especially your protagonist, should have something that they want. Unless you want to have a great degree of difficulty figuring out how to fill up screenplay pages, you should use that goal as a guide to figure out the active steps they take to attempt to achieve it.
This is where GTD comes in. To greatly oversimplify the system, you first determine a general picture of what a successful life looks like. You then determine what concrete things you would need to achieve to make this general idea your reality. These are your projects; your goals. With each project comes a series of smaller tasks you must accomplish in order to complete it. GTD helps to order these tasks so you can easily determine which one is the next action you should take.
Apply this process to your characters. GTD can become a character-based form of outlining, where you dig into their desires and look for potential scenes that best show their pursuit of these desires. Is there an action that they need to take that might be particularly difficult? Is there something essential to their being able to continue that they need to accomplish first?
A GTD outline for your character can also become a checklist to follow along with as you revise your script. As you read through, mark off the tasks that your character is accomplishing. See what’s been missed. See what else they might need to do.
Your character doesn’t necessarily need to have access to this list, but you should. In helping to determine potential actions and pitfalls for your characters, knowing the best path for them to take will help you create interesting detours. It will help you keep track of things your character may have forgotten about, things they don’t want to have to do (but must), or new problems that they need to tackle along the way.
And there’s nothing that says you can’t use the system for yourself while you’re at it.