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While the lighting, setting and camera shots help to convey the film's theme, I find the actors the most effective element for doing this in the movie.
A theme is:
...a common thread or repeated idea that is incorporated throughout a literary work. A theme is a thought or idea the author presents to the reader that may be deep, difficult to understand, or even moralistic. Generally, a theme has to be extracted as the reader explores the passages of a work. The author utilizes the characters, plot, and other literary devices to assist the reader in this endeavor.
The theme that stands out most for me is Atticus' belief that to truly understand someone, you must walk in his or her skin.
This is seen throughout the movie as Atticus demonstrates compassion, sympathy and kindness to others, regardless of how they treat him, and regardless of a person's skin color. Atticus is able to see the world through the eyes of others, and therefore show kindness to them even under the most severe circumstances. For example, Atticus reminds the children to leave Boo Radley alone, understanding that Boo is entitled to privacy, perhaps especially because his life has not been easy for him.
One scene in the movie that best demonstrates this theme is the end where Jean Louise comes face-to-face with Boo Radley at Jem's bedside. Atticus quietly and respectfully introduces Scout and "Arthur" as if they were meeting over tea. Atticus shows kindness and deference toward the ghost-like character of Boo Radley as he presents him to Scout. And in the same fashion, Scout takes Boo's hand to lead him out of the room, onto the porch, and ultimately home again.
The actors convey the importance of understanding and respect for others, taught by Atticus, and carried out at the end by young Scout.