I'll use Diversion from my own work as an example. With the Diversion crew, Bo and Lucky were both raised in the South, but Bo spent four years in the Marines, and holds a pharmacy degree, meaning eight years of higher education. They don't sound the same, but sound closer to each other than to their boss, Walter, who comes from Boston and, though it's not yet been said in the books, he grew up in an affluent family. Lucky also has a seriously bad attitude, and wears his prison sentence like a badge of honor. Yes, he swears a lot. He's also our sole POV character.
I've been taught to rate words on a monetary value. Words like "conversation" I price at $20, "can't" is $5. Lucky uses five dollars words. While Walter most certainly uses a lot of simpler words, he also peppers his speech with terms that are more sophisticated. Bo or Lucky might say something is "cool", while Walter might label it "extraordinary". It would be out of character for Lucky to say "extraordinary" or for Walter to say "cool". Even without tags, you should be able to follow the conversation, based on what is said, like my friend, who is from Upstate New York, sounds different from Southern me.
Trust me, readers (and my betas) know when I cross lines. In the case of my betas, I'll get notes about OOC (out of character).
Here's a bit of their interaction.
Walter said, “Nicely done this weekend, Lucky. As usual you went over the top with the stealth and theatrics, but Bo wasn’t able to track you.”
Not surprising. No wonder Walter had ordered Lucky to go all out. He reckoned it said a lot about his skills when his boss used him for a training exercise. “Who’d you put him with?”
Partnering Newbie with an imbecile like Keith reduced the likelihood of skill alone rendering Lucky untraceable. “The jerk-off can’t tell his ass from a hole in the ground. You’d have done better to saddle the kid with…I mean, assign him to someone else. Maybe Art.”
Walter sniffed, perhaps at Lucky for taking a potshot at a member of “the team” he took such pride in. Teams were for sports. Lucky worked alone.
“Actually, his methods were pretty thorough.” Bo blew into his cup at a light green liquid; a fresh-mown hay odor drifted across the table. Lucky wrinkled his nose. Real men drank coffee—black.
“Uh-huh, and exactly how much experience do you have to compare it with?” Lucky gleefully rubbed in Bo’s wet-behind-the-ears-job-wise status.
“You have to excuse Mr. Lucklighter,” Walter interjected. “He’s not known for playing well with others.”
“Only because others don’t play well with me. It’s not my fault if half the team thinks I’m an ungrateful wretch who should wake up every morning kissing your boots and making up for past sins.” Lucky shifted in his seat, directing his muttered comment to Bo. “Keith believes I should be ashamed of my mistakes. Why should I? Especially when boss man here”—he hiked a thumb at Walter—“uses my mistakes to full advantage.”
“Now, Lucky.” Walter rolled his eyes and heaved out a dramatic sigh. “Your teammates have your best interests at heart. If you made more of an effort to get along with them, you might discover they’re pretty decent people.”
Lucky snorted, Walter ignored him. “Bo graduated top of his class from Virginia College School of Pharmacy, and he completed his first assignment in record time.” Walter gave his best “Proud Papa” impersonation.
Lucky’s hackles rose. A guy on the job for less than two months already had Walter eating out of his hand? “Well, tell me about it, since I’ve been out of the loop this past month, babysitting idiots who deserve the reaming they’re gonna get.”
Bo sat his cup on the table, the better to have both hands free to gloat with. “Nothing much to tell. In fact, I found the whole exercise pretty unremarkable.”
The trick is to always make Walter sound like Walter, while keeping Bo and Lucky in context too. And as each new character is added, they, too, must have a unique voice. Even for minor roles.
Now that's you've gotten a feel for Lucky, let's take a look at another scene, the first with him in character:
“Pull out?” Bo worked hard and devoted long hours to the assignment. It seemed, after the “wannabes” of Diversion Prevention and Control put their collective asses on the line, the big boys intended to waltz right in and take over. While Lucky didn’t personally give a shit for himself, his partner deserved better, deserved the kudos taking Ryerson down would earn. Had he really insisted that they get Bo out of there a few days ago?
“What gives them the right to call the shots? We’ve been here from the get-go, busting our asses.”
“We’ve done our job, gathered enough evidence to warrant a raid. We only waited long enough to put the other puzzle pieces in place. Regency Pharma, Rx Dispose, Ryerson Clinic—the nets are cast and about to be dragged in.”
Now with Lucky and Walter out of character:
“Pull out?” Bo had exhausted himself and devoted countless hours to the assignment. It seemed, after the “wannabes” of Diversion Prevention and Control risked themselves for the case, other agencies planned to swoop in and reap the benefits. While Lucky didn’t personally care for himself, his partner deserved better, deserved the accolades for proving the case against Doctor Ryerson. Had he honestly insisted that Walter reassign Bo a few days ago?
“What gives them the right to assume control of our case? We’ve been here from the start, working hard.”
“We’ve done our job, found enough shit to warrant a raid," Walter replied. "We only waited fucking long enough to see how bad they'd screw up. Regency Pharma, Rx Dispose, Ryerson Clinic—they're going down.”
Hee. Actually, that was fun. But I hope I made my point and clarified about characters' individuality, and how confusing it is for readers when they don't have distinct, consistent voices. Also, though characters may grow in the course of a story, an abrupt change in behavior will be noticed and likely labeled a "personality transplant", unless the protag has a life changing experience and does a believable about-face.