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There are definite differences between the movie and the novel, To Kill a Mockingbird, by Harper Lee.
Because the actors seem to so precisely portray Harper Lee's vision of her novel's characters, the climax of the story is particularly effective.
It is the combination of such diverse characters that provide the chilling high point of the story: an assailant, whom the children cannot identify at the time, attacks them on their way home from school after the pageant. It is dark and the children accurately present frightened youngsters who cannot process or defend themselves against the diabolical evil that descends upon them.
Perhaps it is the difference between the two fathers in the film that add to the sense of horror and fear during the attack: Atticus is extremely honorable and well-intentioned, and his is the strong guiding hand that has shaped the world into a safe place for his youngsters.
On the other hand, Bob Ewell, whom we discover to be the attacker, is a violent and uncaring father and man. As the very different worlds of these polar opposites collide, reality for the children takes on an ugly and horrific face. The innocence of the children, facing the evil of Mr. Ewell, strikes at the heart of the audience. [It is only through the person of Boo Radley--who has the innocence of a child, but has himself been mistreated by his uncaring and abusive father--that salvation is at hand, and the Finch children are delivered safely back home.]
There are certainly other aspects of the film that contribute to the effective quality of the movie's climax, but I find the actors most effective in doing so.