The film To Kill a Mockingbird follows closely to the book but does carefully reduce the content to demonstrate the theme of racial prejudice.
One technique used to focus on racial prejudice is the focus on the trial. The book has a lot of additional information, but the movie cuts much of it out. For example, the trial takes up a good portion of the movie’s runtime, and scenes like the Christmas incident are left out. At the time the film was released in 1962, racial tensions were high. Atticus’s gentle nature and ambitious defense of a black man is a key scene.
Another technique the film uses is to quote the book very closely in some places. During the trial scene, many lines were lifted from the book to further emphasize the issues of race. In some cases, lines are adapted from some parts in the book and used for effect later. For example, Atticus tells his brother that he is concerned about what Scout and Jem will see.
What bothers me is that she and Jem will have to absorb some ugly things pretty soon. (ch 9)
In the movie, Atticus says this to Jem after Bob Ewell has accosted him.
There's a lot of ugly things in this world, son. I wish I could keep 'em all away from you. That's never possible. (imdb.com)
This technique allows the filmmakers to use more of the material and really drive home their theme of racial injustice.