The story is about your hero overcoming obstacles to reach a goal.
In the wonderful heat of creation, as you begin to add dimension and complexity to your story, never for a moment forget that your hero is the main character and belongs on center stage as much as possible.
One way to assure this is to connect your subplots to the hero in some way. If they don't directly involve the hero, perhaps they involve friends of the hero - what Jarvis would call "the hero's helpers."
And as you let us see the antagonist/villain, remind us that the "villain's goal" is to stop the hero.
Most screenplays that fail break down in Act Two - and the problem often has something to do with focus, with the story spine getting lost in action too far removed from it.
Be aware of this potential problem area as you move through the first half of your second act. Keep reminding yourself that the story spine is the story spine is the story spine: a hero facing obstacles that stand in the way of reaching a goal.
Keep on track - even as you follow where the story wants to take you. If it takes you too far adrift, just take pause and say, "Well, that was fun," and back up and start over - and get back on track.
One reason the screenplay is more naturally a form for tree people is that there are such rigid demands on every section of it. In a form this efficient, where so much is done with so little language, there's not time for the kind of "literary grooving" that forest people get attracted to.
Don't stifle your creativity - but also don't be afraid to erase pages and pages of writing as being irrelevant to your task.
Become your own best/worst critic and task master. Force yourself to keep on track with your story.
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