1. Dialogue is the last resort. Use anything else to tell your  story before you resort to dialogue. You might often hear of actors on the set who look at a whole paragraph of carefully crafted dialogue then  turn to the director and say “ I can do that with a look.” They are usually right
  2. Listen. Many new writers often say “ I can’t write dialogue”. What they really   mean is that they have not developed an ear for naturalism. They have not honed the art of listening to what people really say. So. listen, listen, and listen some more. Every human being in the world is a master of writing the dialogue of their own narrative. It is the screenwriters job to learn how to create those characters in their head ,  who can  literally tell them how to “write” their own  lines. So listen to what the real folks say.
  3. Differentiate. Every character in your screenplay should have a distinctive “voice”. You would never cast clones in a screenplay, so why do inexperienced writers make all their characters sound the same? It’s because they haven’t really grasped the fact that no two people ever look or talk the same way…unless it’s a Kraftwerk  biopic you’re writing, or the opening speeches at a North Korean Communist Party rally.
  4. What do they want? People talk for a whole variety of reasons, but a very good way of fleshing out your first draft is to ask yourself what each character’s wants are  in the scene, and what are they going to say to get what they want.
  5. Hide the truth. Human beings rarely say exactly what they mean. Indeed, the classic moment in many screenplays comes at  the second act turning point where for the first time in the entire script, they finally DO say what they mean. “The truth? You can’t handle the truth!” So…don’t write on the line…write round it, write under it…write over it…and choose very carefully the point in your screenplay where your characters finally tell it how it is. It will be a powerful moment if they’ve been hiding it for the past 4 reels.
  6. Information is not enough. If all your lines are about is providing information, then you might as well use graphics or a dancing dog in the background with a sign round it’s neck. Dialogue should be part of the action, part of the character, part of the forward movement of the narrative. Write dumb reportage as dialogue and the director will cast dumb reportage actors. You want your lines to be spoken by the best actors available, so give them something more than “ Excuse me sir there’s a phone call for you “.If it’s only information, think of another way of providing it rather than dialogue.
  7. Don’t tell us what we already know. Dialogue is not there for the good of the characters, it’s there for the audience. If you just saw the heroine pistol whipping the bad guy and tying him to a passing vehicle with a  barbed wire lasoo, don’t start the next scene with her telling her boyfriend  “Hey, I just kicked the Mekon’s butt and sent him down the highway on the back of a Garbage Truck”. We’ve seen it , we don’t need told it again. I was once informed by a very bad writing coach that I should “Tell them, tell them what you’ve told them, and then remind them what you told them.”  Nonsense. Make the audience work. Make them listen. Tell them once in dialogue, and then use images, tone and action to underscore it, not more dialogue.
  8. Keep it lean. Think of words like money in a skinflint’s bank.Don’t spend a single penny more than you have to . Unless your character is a verbose chatterbox, use as few words as possible to tell the tale. No successful screenplay  ever used too FEW words. But lot’s of mediocre ones have far too many.
  9. Sharpen your sword. Good dialogue is like a fencing match. Attack, parry, riposte, with the final stab right at the heart of the opponent. Even if they are getting along, characters should be constantly vying for supremacy in the cut and thrust of crackling dialogue. Think of Bogey and Bacall , Curtis and Lemmon, McKenzie and Patterson. Who are the last two guys? I heard them ripping the proverbial  out of each other at the game the other night. Those guys could WRITE!
  10. Surprise yourself. Never trust the first idea. Sure, write it down in your first draft, but when you go back and read it over you will often realise that it’s pure corn fed cliché. The girl’s  in love?…have her  say “I hate you.”  And then kiss the guy. Result.