The most common mistake in writing dialogue is making every character sound alike. The second most common mistake is making dialogue boring. So to fix both problems, try one of these techniques.
First, word processors (either ordinary ones like Microsoft Word or specialized ones like Final Draft) make it easy to search and replace text, so one simple technique is to name your characters after people you know.
For example, if you have a co-worker who’s particularly obnoxious or admirable, use that person’s name for a character. Now when you write that character’s name, you’ll always think of that real person and imagine how they might say something.
A typical dull dialogue might read like this:
BOB: Hey, what’s up?
JANE: Nothing much. What’s up with you?
BOB: Just wondering if you’d like to get a drink?
Now suppose Bob is someone you know who’s an obnoxious jerk and Jane is someone you know who thinks she’s better than everyone else. Knowing this, you can imagine dialogue those real people might say:
BOB: I normally don’t do this, but today’s your lucky day. I’m free for the moment to share a drink in honor of my manliness.
JANE: I’d rather slit my throat than be seen with someone like you.
BOB: And I’d rather be seen with a rotting sheep carcass than lower myself standards to be with you, but since you seem to be alone at the moment with no one else around, I consider it a charitable donation to share my time with someone like you.
JANE: You’re really full of yourself, you know that?
Notice that by relating your characters to real people you know, you can easily imagine dialogue they might say. Don’t be afraid to go over the top with your dialogue because you can always scale it back. The point is to use real people as a tool to help you write more interesting dialogue.
A second trick is to identify each character with their dominant characteristic. So rather than call someone BOB, call them ARROGANT SOB in your screenplay. Now instead of trying to imagine what Bob might say, it’s much easier to imagine what an Arrogant SOB might say in any given situation.
By substituting real names or descriptions for character names, you can more easily identify how to write unique dialogue for each character. Then simply use your word processor’s search and replace feature to replace those names with the actual character names you want to use later.