What are the differences between how well racism is presented in Harper Lee's novel To Kill a Mockingbird as opposed to in the film?

In the novel To Kill a Mockingbird, author Harper Lee spends a great deal of time developing the theme of racism, whereas in the film of the same title, racism is only briefly mentioned.

In the novel, Lee develops the theme of racism alongside Scout and Jem as they mature. The more they mature, the more they learn about racism. More specifically, they have multiple experiences that tell them about the racist world, aside from learning about racism from Tom Robinson's trial and from slanderous comments about their father given by kids at school and people like Mrs. Dubose. For example, Scout learns about racism as she joins the ladies of her aunt's missionary circle for refreshments and hears them hypocritically speak out for the needs of the African tribe called the
Mrunas while simultaneously saying the African Americans of their community need to stop grumbling about social injustices and be content in their rightful places of subordination. Scout also learns about racism from hypocritical comments made by her third-grade teacher protesting against Hitler's treatment of the Jews while also saying the African Americans of their community need to be put in their rightful places of subordination.

In contrast, many of the above scenes do not take place in the film. Instead, other than the slanderous comments from kids at school, Scout and Jem mostly learn about racism during the trial. In fact, it is during the trial scene that the theme of racism is developed the most. In the film, during Tom Robinson's cross-examination, when asked by Mr. Gilmer why he spent so much time helping Mayella Ewell, Robinson makes a very fatal statement:

I felt sorry for her because ...

He is never allowed to finish his statement because Mr. Gilmer and the spectators of the court are in an uproar that he, a black man, should feel sorry for a white woman. When we feel sympathy for another person, as opposed to empathy, we automatically place ourselves in a position of superiority over that person because it is that person, not us, who is in the terrible circumstance. In the segregated South, the African American should feel inferior at all times. Hence, the film implies that Robinson's statement of sympathy sealed his fate--it is the primary reason why the jury returned with the guilty verdict. It is also through Robinson's expressed feelings of sympathy and the outrage his feelings provoke that the theme of racism is developed the most in the film.

In contrast, while in the novel, Robinson does make a similar statement and Mr. Gilmer does negatively comment on it, it's not evident Robinson's statement of sympathy alone sealed his fate. In the novel, Lee asserts that his fate was sealed the moment "Mayella Ewell opened her mouth and screamed," and nothing Robinson said or could have said would have changed that (Ch. 25).

Hence, as we can see, Lee spends a great deal of time developing the theme of racism along with the children's changing and maturing perception. In contrast, in the film, the theme of racism is developed primarily during the trial scene.