Dialogue has to show not only something about the speaker that is its own revelation, but also maybe something about the speaker that he doesn't know but the other character does know.
Eudora WeltyOnce you're into a story everything seems to apply-what you overhear on a city bus is exactly what your character would say on the page you're writing. Wherever you go, you meet part of your story.
Eudora Welty