The quietest characters in the novel are Atticus, Heck Tate, Miss Maudie, and Arthur "Boo" Radley. Atticus does a lot of talking, of course, because he's a lawyer, but he isn't loud or belligerent (compare him to Mr. Ewell and Mrs. Dubose). He has a quiet, meditative style, and doesn't let anything ruffle him. When the whole town is talking smack about him "lawing for niggers," he ignores them, treats them with respect, and tries to set the example for his children in hopes that they'll learn to understand that people have opinions--sometimes nasty opinions--but they still are good people inside. Moreover, they're their neighbors, and they will go on living with them when the trial is ancient history.
Heck Tate doesn't say much, even though he's a recurring and important presence in the novel. After the attack on Jem and Scout and the death of Bob Ewell, he thinks about the situation and decides unilaterally that Ewell "fell on his knife." He does this to save Boo from the limelight, because to praise such a man as a hero would be cruel.
Miss Maudie is also a woman given more to listening and thinking than talking. Her strength is obvious in the women's missionary society meeting when Mrs. Grace Merriweather subtly badmouths Atticus (in front of Aunt Alexandra) and she calls her out on it ("His food doesn't stick going down, does it?"). When Mrs. Merriweather pretended to not know what she meant, Miss Maudie simply said, "I'm sure you do"--and her point was made.
Of course, Boo is the quietest character in the story. The only words he utters in the entire book are to Scout, after he has gone to see Jem: "Will you take me home?" But he is there for the children when they need him, protecting them from Bob Ewell's homicidal rage.