Ask why the character is behaving that way. Ask them what the people who made the movie wanted them to think about the characters, and how they could tell. This not only teaches them about narrative and point of view, it helps to teach them critical thinking as well. Many classic children's books have been made into movies. A child who has learned about Abraham Lincoln, Thomas Edison, Malcolm X, Charles Lindburgh, or Helen Keller in school can see movies about their lives.
Children also love movies that feature a child as a major character.
Some children get attached to particular actors and will want to see all of their films. Movies can teach a child about the careers or backgrounds of family members or friends. Older movies do have the advantage of telling their stories without the kind of language, violence, or nudity that led to the development of the rating system in the late 1960's. But the disadvantage is that they sometimes reflect assumptions or attitudes that are inconsistent with modern views, particularly about women and minorities. This creates an important opportunity for discussions with the children about those attitudes, about the history they were a part of, and the history since, and, most important, about being able to identify prejudice and its impact, to make sure it is eradicated.