Self-Adaptation: During the filming of To Kill a Mockingbird, Harper Lee used to come everyday to the set, but stopped after three weeks because by then she knew the movie would be fine without her.
Throw It In: In the film, Tom Robinson's emotional breakdown during his testimony was not scripted.
What Could Have Been: Originally Lee wrote a book called Go Set a Watchman about an adult Scout coming to terms with life in the South as the Civil Rights Movement begins to pick steam. However, her publisher/editor was more intrigued by the childhood flashbacks and asked her to rewrite the book based on those scenes, which became To Kil A Mockingbird. Decades later, the manuscript for Go Set a Watchman was rediscovered and published.
Universal wanted Rock Hudson to play Atticus in the film, but he was deemed too young for the part. Spencer Tracy was the director and producer's first choice, but he was unavailable. James Stewart was also offered the part, reportedly second, but told the producers he believed the script was "too liberal," and feared the film would be controversial.
Write Who You Know: Scout was based on Harper Lee's childhood, Atticus on her lawyer father, Jem on her four-year older brother, and Dill on her childhood friend Truman Capote. Boo Radley was based on one of her neighbors, and Tom Robinson was the amalgam of several cases. Ultimately everyone in the book is based off of someone from Lee's childhood. The townspeople were not amused. However, the book was Vindicated by History in Monroeville (Lee's hometown). They perform a stage version of it every year with Audience Participation for the courtroom scene: the audience is segregated as they were in the courtroom, and twelve men from the crowd are selected as the jury.
Fun Fact: The movie adaptation is Superman's favorite movie.
Life Imitates Art: At a couple of points in the movie, Scout, who is only just learning to read, sits on Atticus' lap to visually follow along as he reads aloud. Decades later, real-life educators began recommending this very technique to parents who wanted to help their youngsters learn to read.
Woolseyism: The Italian title of the book and movie is 'Il Buio Oltre La Siepe', 'The Darkness Beyond The Hedge', which is a pretty poetic description of the main theme of both works; not just racism, but also fear of the unknown, whatever is in that darkness that is just beyond the hedge that borders the world we do know, like Boo Radley.