What makes good dialogue is constantly under discussion. People say they like dialogue that sounds like how people actually talk, but if you watch enough reality tv you’ll soon find out that unscripted dialogue from the mouths of “real” people is usually terrible. It’s usually repetitive and completely lacking in subtext. The idea is forced confrontation, but in stirring the pot, the directors fail to play out the drama in full and give depth to their players. Thus reality tv is shallow, forced and ultimately boring. Really entertaining dialogue is hard to write too. You can try to hard to be witty, or not hard enough and imitate something you thought was funny when somebody else did it. You come up with a different kind of forced dialogue. Bad sit coms and commercials are rife with these types of performances, trying to cash in on a popular character by attempting to duplicate that style of talking.
As much as you want to get away from clichés in your storytelling, you should also avoid them in dialogue. Right off the bat I can tell you three really bad lines: “Really?” “I know. Right?” and “I’m not gonna lie.” Real life people memorize scripts to. they repeat lines like this in everyday situations without thinking. You may hear real people say it all the time but people should not be saying them in your movie. Try to create the next thing people will constantly spew out without thinking. You’re a writer, dammit.
That’s not to say that you shouldn’t listen. You should. A lot. You should spend a lot of time around people and really here the words they use. Even better, go to where the type of people in your script are and hear how they talk. Watch what they do to. spy on people. take notes of distinct and even odd behaviors. those are the things that are interesting and you may draw from that at some point. As for dialogue, the idea is not so much to be realistic as it is to be natural. You want to take the reality and give it a stylish flare that will have people believing, but also captivated by the gems spilling out of your characters mouths.
In reality shows, these poor people can’t say a single interesting thing. That’s why scripted film and television exists. Not everybody is the cast of Curb Your Enthusiasm. and those that have improve training are in a sense, writers. Actually, that’s exactly how I got into it. Samuel L. Jackson has a gift for taking scripted words and making you believe they just flowed naturally from his brain to his mouth. Tarantino writes incredibly stylized yet natural sounding dialogue. Kevin Smith, had a hard time getting the natural to pair with his style until he found Jason Lee, who is his Samuel L. Jackson. Suddenly, those unnatural words have a form and the breath of life where even Claire Forlani couldn’t get them to work.
The point is that you are a student of the language and you want to be informed by culture and conversational art. You want to have as much truth to draw off of as you can, but you are also charged with making your story entertaining. You must intrigue and delight your audience and dialogue is one of many tools to make this happen. You can write with your own voice and leave it to the actors to give that voice substance. You also have actors in your head portraying your purest vision of the characters. you have a sort of baseline for their sort of speech. As a writer, preparing a script you are under considerably less pressure than an improv artist and more is expected of you in return, so give those characters in your head direction and fish for fresher choices. You already have the scenes mapped out so the dialogue is where it will all come together and your script will take on some life.