Dear Fellow Writers,
"Y'all" is the second person PLURAL pronoun in Southern US English. Plural. Not singular. Which is only logical, when you consider that it's a contraction of "you all."
You may think you've heard a Southerner use "y'all" in the singular, but this born-and-bred Alabamian believes you are mistaken. If a Southerner asks her friend, "Are y'all going to the game Saturday?" she doesn't mean, Are YOU, the one person I'm talking to right now, going to the game? she means, Are you, Bobby, and the kids all going?
I've met a few people who think "y'all" is singular and "all y'all" is plural. No. "All y'all" either refers to a large group or is sort of a stand-in for "everybody" when you're trying to get a group's attention. As in, "If all y'all will gather over here, the photographer is ready to take the picture," or, "All y'all should come downstairs now--the ribs are ready."
Note too that it's spelled "y'all," not "ya'll." And please don't write "ya" instead of "you" when your Southern character is using second person singular. To my ears, Southerners are no more likely to clip the "ooh" sound off "you" than any other speakers of American English. If anything, since some Southerners really do talk a bit slower than their Yankee brethren, they're more likely to fully sound out "you."
Yes, I'm in Her Grace, the Duchess of Pedantry mode here. But badly written Southern dialect is nails on a chalkboard to me. Actually, it's one of the reasons I don't write dialect--or, at least, I don't use misspelled words to indicate accents--for any of my British characters. I try to avoid anachronisms or Americanisms for all my characters, and I use diction and word choice to show differences in class, education, and personality. But having read so many non-Southerners botch my native accent, I don't trust myself to write Cockney or Scottish or whatever in a way that won't turn native speakers against me.