Think about how the kids react to any perceived presence of Boo Radley, and the stories they tell each other about him. They tend to build him up like he's a mythical monster to the point that they become afraid of him without any personal experience to legitimize this reaction. That's the way children's minds work; their imagination runs wild and turns regular people into fantasy.
Now think about how Atticus and Miss Maudie regard Boo. They use his real name. They tell the children to leave the poor man alone. They shut down the gossip. And at the end of the story, Atticus is willing to lie, something completely against his normal character, in order to protect Boo from further persecution by society in thanks for Boo saving his children.
