What Could Have Been: Originally Harper 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 Kill a Mockingbird. Decades later, the manuscript for Go Set a Watchman was rediscovered and published.
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 (Alfred "Son" Boleware, who was put under house arrest by his father well into adulthood after a teenage vandalism incident), 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.
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.
Creator-Preferred Adaptation: Harper Lee was on set for the first day of filming and was reduced to tears by how well Gregory Peck embodied Atticus Finch.
Deleted Role: Ruth White's role as Mrs Dubose was heavily cut down, because producers felt her scenes slowed the pace down.
Dyeing for Your Art: Robert Duvall stayed out of the sun for six weeks and dyed his hair blonde, for the role of Boo Radley who, according to the story, spent much of his life as a recluse.
Hostility on the Set: Mary Badham (Scout) and Philip Alford (Jem) didn't get along while filming. She messed up nearly every take in which the family eats at the table. He didn't like eating the same meal dozens of times, so in one of the takes of the scene, in which he rolls Badham in the tire, he aimed it at an equipment truck in an attempt to hurt her.
As adults however, Badham has stated that she and Alford get along fine now, and remain good friends.
I Am Not Spock: Mary Badham chose to retire from films after this, so she remained as Scout in the public consciousness. Even Gregory Peck - with whom she remained close friends - continued to call her Scout as an Affectionate Nickname.
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.
One-Take Wonder: Atticus Finch's closing statement, in which Finch demanded the jury "do their duty", was originally planned to take several days. But Gregory Peck somehow managed to nail the speech in only one take—that alone was enough to guarantee Peck's first (and only) Best Actor Oscar.
Playing Against Type: Brock Peters had mostly played villains before this film. Here he plays the completely innocent Nice Guy Tom Robinson.
Self-Adaptation: During the filming, 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. Brock Peters spontaneously started tearing up while filming.
Vacation, Dear Boy: Philip Alford only auditioned for Jem when his mother said he'd get to miss a day of school.
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.