There's a book called If You Can Talk, You Can Write by Joel Saltzman. The premise is that if we're able to relate information vocally, then we can write it down for others to read. Unfortunately, many people write in stiff, forced, and uncharacteristic manners, like they're "trying to write." Saltzman continues that if we imagine we're just talking to someone when we write, the resulting words will sound more natural, conversational, and free-flowing.
As scriptwriters, we take this premise one step further: If you can talk, you can write what others will read, memorize, and speak. The end result is we want others speaking our words in clear, understandable, and interesting ways. The audience needs to comprehend our intent the first (and only) time they hear the words. Our characters should speak in non-predictable ways so the audience stays attentive.
I say non-predictable as opposed to unpredictable. Unpredictable means you don't know what will happen and things could take off in random directions. Non-predictable means the audience won't be lulled into boredom because they anticipate characters' familiar speeches and clichéd sets of words. Rather, the dialogue will surprise and delight them.
There is a saying about writers who can "turn a phrase," who can build upon what exists to create new ways to state things, or are able to modify existing phrases to make them new and fresh. Newspaper headlines offer daily examples of this, where something familiar has been modified by something else to create a clever variation.
Malaprops, named after Mrs. Malaprop, from Richard Brinsley Sheridan's The Rivals, are fun to play with. Mrs. Malaprop mis-spoke nearly everything, changing the meaning of her speeches by her mis-understanding of the English language, and substituting one word for a similar sounding, but vastly different, word.
Other ways include taking well-known phrases and tweaking them with something like, "See a penny, pick it up. All day long, you'll have ... a penny." The audience is waiting for "good luck," but gets a surprise instead.
As you listen to the world's conversations and gather your material, carefully observe those instances, whether by fate, accident, or some other cause, where what you expect a person might say in a singular situation, instead comes out differently, and with more interest. You are surprised and delighted.
This is what you're trying to achieve for your listeners, your readers, your audience; this is the scriptwriter's goal.
