A film has less time to tell its story and therefore often concentrates the events of a story into fewer characters; when a book makes the transition to film, characters and their actions are often combined. For instance, Miss Stephanie Crawford is Dill's aunt and Cecil Jacobs, not Francis Hancock, drives Scout to break her promise to Atticus about fighting. Aunt Alexandra isn't present in the movie at all, so the issue of Scout "acting like a lady" never plays a major role in the film.
Film also often introduces new characters to help develop the story line. In the film, Scout and Jem have a conversation about their deceased mother which brings her alive for the viewers; the book devotes a single paragraph to her. Viewers also meet Tom Robinson's children and father. His father isn't mentioned in the book, and his children receive only a brief mention.
The benefit of film is that viewers get to see the characters. They can put a face with a name, so to speak. And characters can say things with facial expressions, hand gestures, and posture that an author must describe to readers. Many people enjoy the advantage of being able to visualize a character; however, viewers can be thrown out of the story if the actor playing the part doesn't fit the reader's vision of the character. For instance, the actress who plays Miss Maudie is thin, much younger, and more conventional than Scout describes in the book, which takes some of the bite out of the character. On the other hand, Gregory Peck, by Lee's own assertion, is the perfect embodiment of Atticus Finch, which gives the character a far greater depth than the book, alone, can provide.
