Some old screenwriting advice: give characters distinct voices. You should be able to read a line without dialogue attribution and know who said it.
Practical example: If I made a list of great George lines, Jerry lines, Elaine lines and Kramer lines, you could probably tell whose was whose. You could tell even if you hadn’t see the episode they were from.
The modern spin on this advice: use your screenwriting software’s character report to generate an entire list of all their dialogue, out of context (this is one of the only things that Final Draft does pretty well). You’ll probably see a couple great lines, but a bunch of disposable ones:
“I could really use use the money.” “Do you know the way out?” “Sharon, she’s my wife.” “Um…yes. I–”
Every character is going to have a few of these, but if most of your character’s dialogue is that bland, odds are you have a bland character. Theoretically, all characters have traits. These traits are best expressed through dialogue.
A THUG “Fuck you, pay me.” “If you know the way out, tell me now.” “Sharon. My fucking wife.” “Hell yeah. I dunno.”
A ROCK STAR “I’m not saying it’s about the money, but it’s about the money, mate.” “If you know the way out, I will totally hook you up.” “Sharon… you know, my current wife.” “Maaaaan….”
A CREEP “Pay me. My body yearns for it.” “If you know of an exit, well, I’d use it for my purposes.” “My wife Sharon. Can’t masturbate forever.” “Ooooh.”
Obviously, not every line needs to turn into a Whose Line bit, some lines are better plain. But if you never color your dialogue, your characters never get colorful. How many times have you read a script where TIM (22) is introduced as cocky and funny, and yet he talks exactly like SCOTT (23) nerdy, all business? Don’t do that.
TL/DR: Find what works about a character and find ways to do more of it. This is a form of patternizing (to make conform to, reduce to, or arrange in a pattern).