Do their speech patterns and physical designs just come up clearly for you when you first imagine the character or did you have to sit down and work them out?
Some of my characters, like Verochka Belova and Jimin Lee ("Hushabye Prince.") and Mrs. Moon ("Damsel in the Red Dress.") self-inserted themselves into the story as fully fleshed-out characters. Their personalities, and thus, their way of speaking, physical appearance, and such, were all pretty clearly defined from day one.
Other characters I learned more about as I wrote them, and some were very active attempts to work out the nuances of the character's behavior so that they would be distinct in the story.
I definitely think the way characters speak should be representative of who they are, and so it shouldn't be something easily interchangeable between characters. The exception in my writing is with certain characters like Kattar and Alicia ("Damsel in the Red Dress") and Hallie and Madeline ("Hushabye Prince") who being best friends, have picked up bits and pieces of each other's personalities, though Hallie and Madeline have grown a lot more alike than Kattar and Alicia ever have.
Kattar and Alicia being two peas in a pod
“Oh.” His lips part - eyes getting a little rounder than usual. “Oh, y-yeah. I think so. I’m supposed to be released Wednesday, and Utkaresh is gonna drive me home. It’ll be so nice to be in my own clothes again.” He adds with a cross between a laugh and a sigh.
“You must have been dying stuck in that white dress for all this time,” I can’t help teasing, “It’s not even stylish.”
He smiles roguishly, and I realize too late that I’ve walked right into the rebuttal-
“Yeah, it looks kinda like something you would have bought.”
Jinwoo and Jimin being the near polar opposites they exist as