How to write good dialogue, making characters seem real and involved in the story. Common mistakes to avoid.
It's a talkie
In most movies there is sound. Sound design and the ability to make a world seem real with Foley is well outside the screenwriter's remit, but what the characters say is defined in the screenplay. A screenwriter must get a handle on dialogue.
So how do we go about writing dialogue? Well this is going to sound insane, but actually, you don't write the dialogue.
Remember by this point you know what happens to every character at every moment, you know what's going through their minds, you know what they want. So essentially the action is simply what they do and the dialogue is what they say. If you've built the screenplay methodically you know exactly what they say at each moment, all you have to do is pick the words to suit the character.
Well, that's a fat lot of help
OK, I admit, that doesn't sound very helpful but it really is. Let me explain. If you're having difficulty writing dialogue it's simply because you don't fully understand what the character wants to say in that moment. If you are having difficulty writing dialogue it means you don't have a problem with dialogue, you have a problem with structure. Go back to the three-acts, take a look at the events, check the way the scenes are constructed. Fix the structure and the dialogue will sort itself out. I promise.
By far the biggest mistake made in screenwriting is exposition. This means characters explaining what's going on rather than their actions showing what's going on. Another example of exposition is when a character explains something to another character even though they both already know the facts.
Don't make speeches long and complex, they're hard to break up in the production of the film and so the director has far fewer options to control the pace.
Don't make characters justify their position unless the scene absolutely demands it.
Allow dialogue to evolve naturally, forcing a favourite saying or joke where it doesn't fit is really awkward.
What makes the good guy good is what he does, not what he says. Feel free to make him cynical, sarcastic, laconic or terse. He can still be the hero. Watch Clint Eastwood in Gran Torino to see the difference.
Audiences don't need everything explained. Show as much as you can manage and say as little as possible.