My daughter posted a meme on Facebook called California English: yeah, no means no; no, yeah means yes; yeah, no, for sure means definitely. That is why I love eavesdropping on random conversations to help with character development. Dialogue can tell the reader so much about the character, sometimes without them being aware of it. If your character answers a yes-no question with “No, yeah,” your reader will see her in a different light than if she answers, “A-yup.”
The next time you are enjoying a ride on BART, a break at your local coffee shop, or waiting for the line to move at the theater, listen to the people around you. Don’t focus on any one conversation, but try to hear as many voices as possible. Airports are even better, as you will get a broader range of accents and dialects. If travel isn’t in your future, call friends and relatives in other parts of the country and concentrate on their sentence structure and word choice. Your characters—and readers—will thank you.