The screenwriting books all say that one of the most common complaints around scripts (of all kinds) is that the characters sound the same, instead of each character having a distinct, unique voice.
But how can you establish distinct "voices" in a script's characters in the first 10 pages? (That's how long you've got, basically.)
It helps, obviously, to have a vivid image, in your mind, of a real-life person (such as a certain actor who fits the part) for each character. As you write, imagine what kind of language that person would use.
The other more-or-less standard bit of advice you encounter a lot in books and articles on screenwriting: Go through and edit all of one character's dialog, whether it's the first 10 pages or the whole script, without looking at any other character's dialog. Then do that for the next character. And so on. This way, you have some hope of achieving consistency in dialect and clean separation of diction patterns from one character to the next.
But how can you achieve characterwise speech differentiation quickly, without resorting to patois or other gimmicks?
I'll tell you a trick I was able to use in the script I just finished. The trick is pretty simple: Have one character mock the other's diction. In my script, Greeners, the hero is a 29-year-old American grad student (think Mickey Rourke 1983). His future partner is a 23-year-old Hindustani woman from New Delhi, a post-doctoral fellow in his department. By page 7, she owes him, bigtime, for putting out a car fire. A few pages later they meet again, and she realizes she never properly thanked him. She tells him so. Then asks: "How shall I repay you?"
No American talks like that. No one says "shall I" in conversation. He says (mocking): "How shall you... 'repay me'? Come on, I didn't do anything."
It's a subtle reinforcement of the fact that they're from two different cultures. She learned British English as a second language. He's James Dean's lost brother. (When he suggests she "thank him" by letting him take her out to dinner, she doesn't nod. She does a little figure-eight head-wobble, Indian for okay/perhaps/why not.)
She's prim, well educated, polite to a fault, and therefore uses proper fully expanded English. Instead of "can't" she tends to say "cannot," for example. He can say "wouldn't've" (as in, "I wouldn't've done that..."); she has to say "would not have."
A few pages of this, and it's pretty clear who's the homie and who's here on a visa.
Another tip: People's true speech patterns always surface in a crisis. One person might shout "Bleedin' Christ!" whereas the next person might say "Oh dear Lord no," etc. So put a crisis on page 5 and differentiate the characters right then and there.
You were planning to do that anyway, though. Right?
If you'd like to see my (WGA-registered) screenplay, drop me a line. My hushmail dot com address is kasthomas. Inquiries held in strictest confidence.