Writing natural dialogue isn't magic. Like every other kind of writing it requires work, but because most people know how to speak they imagine they can write speech and refuse to do the work. Don't fall into that trap and listen to what I've got to say.
"So, what's this problem with dialogue, then?"
"I don't know, but I just can't seem to get it right."
"What's wrong with it?"
"It doesn't read natural. Sort of stilted."
"Did you read it aloud to yourself?"
"Yeah, that's when I knew it wasn't right. It sounds like I'm reading from a textbook."
"Well, some people sound like that."
"Not the characters I write, they're meant to be street-wise."
"Well, you know plenty of street-wise kids, can't you write the way they talk?"
"I tried, but then they just sound dumb. And it seems so contrived."
"So you're stuck between text book and street-wise?"
"Yep, I guess I should write myself, at least I know the way I talk."
"Well, there's your problem. You're observing people and writing their dialogue as you imagine they speak it."
"Isn't that what I'm meant to do?"
"No way, the observer always affects his perception. What you have to do is inhabit your characters."
"That sounds crazy."
"No, it's not. You can't expect your dialogue to read naturally unless you put yourself in the character's head."
"But that would take forever, if I have to keep switching from one character to the next and remember to be myself if I feel like a coffee."
"It just feels strange at first, but pretty soon you'll be doing it as natural as breathing. As you write each character's dialogue you'll slip into their way of thinking and pattern of speech. It's not everybody gets the chance to be half a dozen people in one day."
"I don't know, seems like that way lies schizophrenia."
"Don't worry about it, it's not such a big deal and after a while it becomes second nature."
"I'm not convinced."
"Look, all I'm asking is that you try it. Write some stuff and see how it sounds when you read it out. What have you got to lose?"
"My sanity, I think. I don't know if I want to give myself over to my characters.
"If they're real they'll demand it of you."
"They can't demand anything, they came out of my head."
"Oh, figments of your imagination, not real?"
"How can you expect your readers to believe in them when you don't?"
"I think you're getting a bit freaked out here. This is fiction writing we're talking about."
"I know, and I'm serious about it, just because it's not real doesn't mean it's a game. If you want to write good fiction you have to write good characters with believable dialogue. Otherwise you're just playing at it."
"Hey, I am serious about this."
"Well, do the hard stuff. If it was easy everybody would be doing it."
"You're a hard taskmaster."
"No, I just don't want to see talent wasted by a little bout of laziness."
"I appreciate that. And to show my appreciation I'm going to take you down to the pub."
"Well, at least you don't need to work on your endings."
I left the bar twenty vodkas later and had lost the ability to speak, never mind write good dialogue.