To refresh your memory of the story: Atticus Finch is the widowed father of the tomboy Scout (Harper Lee’s self-portrait) and Jem (Scout’s older brother). Atticus is a lawyer appointed to defend Tom Robinson, a young black man falsely accused of raping a white woman. This event is based partly on a case of her father’s, and partly on the infamous Scottsboro Trials of young black men falsely convicted of raping a white woman, also in the 1930s, when Harper Lee was about the same age as her fictional stand-in, Jean-Louise, known as Scout.
Harper Lee transformed memories of her childhood: her father was the inspiration for Atticus Finch, Truman Capote became Dill, her brother Edwin and cousin Jennings became Jem, and Boo Radley was perhaps based on someone else in her hometown of Monroeville, Alabama (as discovered by the director of a University of Alabama production of the stage version, who visited Monroeville.)
In Monroeville in the 1930s, a rich man’s son was caught joyriding in a stolen car. His father persuaded the sheriff not to arrest the boy, but to leave the punishment up to him. He imposed three years of house arrest, but it turned into a life sentence when even after that time the young man found he could no longer face leaving the confines of his house, except at night. He became an object of mystery and fear in the neighborhood. At least according to the local story.