The courtroom scenes are condensed in the film. Gregory Peck as Atticus Finch delivers a shortened version of Atticus's closing arguments to the jury. The lines he does say are verbatim, but several points from the speech aren't included. Neither does the film explore the aftermath of the trial or portray the conversations Atticus has with his children in trying to help them understand the situation.
The film addresses the plight of African Americans only through the trial. Calpurnia is treated respectfully by everyone, the children never attend Calpurnia's church, and on the day of the trial, blacks and whites enter the courtroom together (although the blacks, and Scout, Jem, and Dill, sit separately in a balcony, just as they do in the book). Remember, though, that at the time this film was in theaters, audiences wouldn't have needed an explanation for these sorts of things. They knew first-hand the challenges African Americans faced. The idea that blacks would sit separate from whites would have been expected — or understood, at the very least — by anyone viewing the film.
Film is very much reflective of the original audience's culture. As a film ages, audiences need more information to fully grasp the story. The fact that the film version of To Kill a Mockingbird is still so powerful is a testament to a fine adaptation of a classic story.
