Some novels present a story form many points of view. Most movies tell only one person's side of the story. Sometime it's easy to use the strongest point of view, or find the character with the most dramatic experience. It depends on which themes the scriptwriter wants to explore.
Most men have no purpose but to exist, Abraham; to pass quietly through history as minor characters upon a stage they cannot even see
If a novelist has created vivid characters, interesting relationships, settings the reader can easily imagine, and intriguing stories, a screenwriter has loads to work with. The challenge comes with deciding what to cut and what to keep.
I always say that the characters in Jane Austen's original books are rather like zombies because they live in this bubble of immense wealth and privilege and no matter what's going on around them they have a singular purpose to maintain their rank and to impress others.
Movie characters rarely get to think out loud or talk very much about their emotions. Instead they have to, very briefly, show their feelings through their action or through dialog.
If a movie has more characters than an audience can keep track of, the audience will get confused and lose interest in the story.
