In chapter 4, the kids played a game that they called rolling in the tire. The boys put Scout in the tire and rolled her down the street. On this particular day, the tire rolled all the way into the Radley yard. When it stopped, Scout grew afraid right away. Later in the chapter she reports her feelings with these words:
I rolled into the Radley front yard. Through all the head-shaking, quelling of nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing.
Scout reported a low laugh to the reader in this instance. Sometimes a laugh can be fun or joyous, but other times it can feel evil. This quote demonstrates that Boo could be an evil man laughing at the misfortune of children.
Another possible quote occurs in the same chapter when Scout, Dill, and Jem are playing the Boo Radley game. As the game is played out, Boo is portrayed as a ghost who haunts the rest fo the family.
Jem parceled out our roles: I was Mrs. Radley, and all I had to do was come out and sweep the porch. Dill was old Mr. Radley: he walked up and down the sidewalk and coughed when Jem spoke to him. Jem, naturally, was Boo: he went under the front steps and shrieked and howled from time to time.