The setting relates to the characters because the setting is a character
Scout describes Maycomb as if it too is a character. She says: “Maycomb was an old town, but it was a tired old town when I first knew it” (Ch 1, p. 5). This tired town is a perfect backdrop for a dramatic story. The story taking place in a small down during the Great Depression in the crucible of the Deep South allows for some interesting characters and events. As the enotes Historical Context page notes, "the South, which was still steeped in its agricultural traditions, was hit hard by the Great Depression" (enotes historical context). The characters interact with the setting, because these characters could really only exist in this setting.
For example, Atticus Finch defends his town against racism. Boo Radley is victimized by the town. The town comes out in full force to save Miss Maudie’s house from the fire. The town attends the trial. The town gossips. The town grieves. In each case, the various members of the town are grouped together as one character.
For instance, consider the story of young Arthur Radley and his gang that terrorized that town.
The town decided something had to be done; Mr. Conner said he knew who each and every one of them was, and he was bound and determined they wouldn’t get away with it, so the boys came before the probate judge on charges. (ch 1, p. 10).
This is another example of the town acting as one, or being described as one. Although the town is acting in a positive way here, this is not always the case. The town also harbors prejudice and racism.
Why reasonable people go stark raving mad when anything involving a Negro comes up, is something I don’t pretend to understand… I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough… (p. 91)
So just as the town provides a setting that produces unique characters, the setting is a character in itself.
For more on the historical context, read here: http://www.enotes.com/to-kill-a-mockingbird/historical-context
Citation: Enotes. "To Kill a Mockingbird." Enotes.com. Enotes.com. Web. 14 May 2012. <http://www.enotes.com/to-kill-a-mockingbird/historical-context>.