Because a film has a limited time in which to tell the story, events from a novel are invariably dropped when the book becomes a film. Although the film version of To Kill a Mockingbird includes every major event from the novel, the screenplay takes place over two years, not three, and many events are left out. For example, the children have virtually no contact with Mrs. Dubose, and the film never shows the inside of a classroom, so viewers don't experience any of the episodes with Miss Caroline, Miss Gates, and some of the other minor characters that create Maycomb's texture and layers.
Lee's novel is a coming-of-age story influenced by a major event in the community and within one family. Scout not only tries to understand and process the trial, but she's also wrestling with the expectations those around her have of little girls. The film, on the other hand, is a courtroom drama that happens to include something about the lead attorney's home life. In its film version, To Kill a Mockingbird only touches on the issues of femininity. The movie never gets into Maycomb's caste system, so viewers don't necessarily know that the Ewells are considered to be "trash."
The implied incest between Bob and Mayella Ewell is never discussed during the course of the trial. Unlike today's films, movies in 1962 weren't allowed to cover such controversial subject matter. Instead, films had to find ways to work around taboo subjects. In this case, the film works around the incest issue by showing Bob Ewell's unscrupulous behavior in other ways. For instance, he begins stalking Jem and Scout before Tom's trial begins, and viewers can see from Mayella's facial expressions in the courtroom that she's frightened of her father.
