A criticism I have toward my writing is I can’t really help noticing my characters all speak the same. Stephen King says the reason he puts a lot of he said she said in his books is so the reader can tell who’s speaking. That’s a given. I’m talking about making characters sound authentic. I tried an exercise writing distinct dialogue without context or story. Just practicing language based on certain character traits. I would just have stereo types having conversations. I focused on who they are as people. Where they come from. Dumbing them down if say they are uneducated. Always trying to bring the dialogue back towards a character who is a narcissist. These are just examples. There’s no real story behind what they were saying. It was just script format with no actions or story.
Old Bitter Cop: “This guy says to me, he says, You know what I hate? He tells me. I hate these goddamn kids and their mobile phones. Cameras and blinking lights. They put a spot light on us and try to destroy our careers. All because these fuckin’ thugs, named after soft drinks with their vibrating cars, tell these kids to say fuck us.”
Spoiled Teen: “Bro can believe my dad won’t give me money for that concert next week. Just because I told my step mom I didn’t have to listen to her because she wasn’t my real mom. He said she cried. Pfft, probably faking, you know? She’s like that bro, for real. She’ll do anything to get her way. Anything!”
I don’t know your writing process but this actually got me out of writers block a couple of times. I took what little bit of dialogue I had and added to them. Creating small stories giving them some sort of form and substance. Plus I thought it was fun because there was no pressure. It’s just stream of conscious. You might give it a try if you find yourself stuck. Pull them out of the main story and have your characters engage one another in white space like the Matrix. Have them talk about anything. Work out how they sound. This advice comes from Quentin Tarantino.