Each character should have their own voice, that is, a distinctive way of speaking that reflects who they are as a person. Each thing they say should be so distinctive that only they could have said it in your story.
A cynical, mature college professor from New York city will speak differently than a naive freshman from the Midwest.
A down-on-his-luck carpenter will speak differently than a real estate tycoon, even though they're in the same industry. They will have different vocabularies and attitudes. Someone who is positive, on top of the world will say things that reflect his world view. The same is true for the carpenter who is worried he won't make the mortgage.
One problem with many newbie scripts is that everyone sounds the same and just about anything they say could be said by any other character.
Make your characters SOUND different, distinctive and you're on your way to a more sellable script.
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