Here are some tips from Pilar Alessandra, an L.A. based script consultant from On The Page. Here are some tips for fleshing out your initial idea into a feature screenplay:
STEP 1: Brainstorming
Find your hook. Nail your story. Define your theme, conflict and genre. Who is your hero?
STEP 2: Outlining
Outline your movie beats and characters (main and supporting). Define subplot.
STEP 3: Scene Inventing
Help your characters tell their own story. Characters drive plot. Define their arcs.
STEP 4: Formatting
Discover your unique writing style. Use caps, space, single word lines and punctuation to convey emotion.
STEP 5: Dialogue Writing
Establish character voice. Add subtext.
STEP 6: Polishing
Use style, format and tone to hone your pages.