*This was supposed to have been posted yesterday, sorry. I had a big history project.*
Voice- what your character sounds like. Word choice, sentence type, intentional grammar mistakes, and regionally acceptable words (like "y'all"). There are more, but that's all I'm focusing on here.
VOCABULARY. This is very, very important. Do not let your happy-go-lucky five-year-old go around using words like "evanescent" and "expurgate". Don't let your Britsh official say words like "potty" and "nappy". Vocabulary is important because it really brings your character to life, and don't just avoid using wrong vocabulary in dialogue - their thoughts and how you're describing your (oops, their) surroundings should include words that they'd know and use. Like, a perky teenage girl might say "sunny yellow", while a depressed girl might descibe the same shade (or any shade) "puke yellow". Look at it from their perspective, through not just through their eyes but through their minds.
SENTENCE TYPE. Simple, compound, complex, compound-complex. I find that not only I but my character prefer simple sentences. Long ones, with multiple verbs, but still simple. And to review/inform you of the types: simple= 1 independent clause, 0 dependent clauses. compound= 2+ Ind. clauses, 0 dependent clauses. Complex=1 ind. clause, 1+ dep. clauses. Compound-complex= 2+ ind. clauses, 1+ dep. clauses. Clauses are a subject and verb, independent means it can stand alone as a sentence. If you have one subject and multiple verbs, they're still simple.
INTENTIONALLY INCORRECT GRAMMAR. Double negatives ("don't got nothing") and perhaps sort of slurring words together ("kinda", "sorta", "alright") is what immediately comes to mind. Don't confuse this with regular mistakes, though. Using the wrong "it's" does not fall under this category. This is kind of regional too, I suppose. But not always.
REGIONALLY ACCEPTABLE TERMS. "Y'all", "ain't", and of course, "soda" vs. "pop". Know the local terms of a place before you try to write this. It's important - like if my character's from where I'm from, it's soda. If she lived in New York, it'd be pop (right?). You might occasionally see me use the word y'all, because it's perfectly acceptable where I live. If I went to New York, they might snicker at me for that same word. This makes your characters realistic. It'd look pretty ridiculous otherwise, especially for the people who actually live in the same area as your character. You don't have that problem with fantasy worlds, but make sure they have special region words as well, to spice it up a little.
Another thing I forgot is SLANG. I don't know if I should include this in that last category, but you know. Your teenager is more likely to use slang, your adult isn't. This one is kind of obvious to me - even fantasy worlds should have slang terms. Not that I like slang, but you can't deny its influence.
This is a quick overview of the topic. You as a writer have a responsibility to make sure your character's voice comes out true. Follow the above, listen to people from that area if it's not where you're from (if you can), at least look it up on Google or make it up as you go along. These are important things that'll enhance your writing. It makes your characters realistic and really brings them off the page.
So, hope this has helped. Y'all have a great day and blessed weekend. ;D