What functions do authors think about when they create characters in their minds or on their pages?
Orson Scott Card, who has written multiple books about the writing craft, mentions that characters can be divided into walk-ons and placeholders, minor characters, and major characters. In a movie like "Les Miserables" or a mystery novel, an author has to consider how much time and description to give to each character he creates.
The walk-ons and placeholders are part of the background of a story. They have their purposes either in large ensemble groups or in small parts. They come in, do their thing, and then leave. In movie credits, such a character might be labeled "second man with gun." In the Sherlock Holmes' stories, the Baker Street Irregulars, a band of street urchins that Holmes relies on for intelligence, is such a group. In "Les Miserables," the priest who gives Jean Valjean the candlesticks or the foreman of the factory who throws Fantine out, help send each character toward his or her destiny. Then the priest and foreman leave the stage/screen, mission accomplished.
In the mystery I just finished writing, Three May Keep a Secret, the county coroner, Dr. Ron Martinez, identifies a fire victim and then escorts her body to the larger city of Woodbury to keep the chain of evidence intact prior to her autopsy. I describe him only briefly, with a few details to show how tired he is when called out in the middle of the night. I also mention he is actually a pediatrician who doubles as a coroner in the tiny town of Endurance. Multiple titles is a characteristic of small towns.
The second category, minor characters, helps the plot along and we learn more about them than the walk-ons. They have their purposes too, and without them the story wouldn't work as well or take some of the turns it takes. In "Les Miserables," Samantha Barbs plays Eponine, a tragic character who is in love with Marius, and she has more screen time than the walk-ons, but she is not considered a major character. It is important that we feel her despair when she realizes that Marius is actually in love with Cosette, because then we understand Eponine's motive in hiding Cosette's letter from Marius. Major plot point!
In Three May Keep a Secret, Grace Kimball has two friends who play supportive roles in her life. One is Deb O'Hara, former junior high school secretary, married with two grown-up daughters, and always ready for a party, especially if it involves margaritas. Deb raises the alarm when Grace goes missing. She is in sharp contrast to Jill Cunningham, another friend who makes infrequent appearances in the book. She is there when Grace learns of an unexpected death and she always keeps the group in focus. She is an accountant and a no-nonsense kind of disciplined person who jogs two miles every morning. Unknowingly, they put Grace in the path of a killer when they volunteer her for a job. They are necessary to the plot, but not major characters.
A sub-category of minor characters is those who provide comic relief, especially in such a tragic story as "Les Miserables." Those characters would be Madame Thenardier (Helen Bonham Carter) and her husband played by Sacha Baron Cohen. In Three May Keep a Secret, two characters offer comic relief. A dark mystery needs to give the reader time to catch her breath after something terrible happens. Ronda Burke is such a character. She is pursuing a career as a stand-up comedian and is Grace's former student. She has an enormously ugly dog she has named "Adonis." Grace's sister-in-law, Lettie Kimball, is also a comic relief character. She believes everything she reads in the tabloids, listens to all the gossip around town, and has outrageous opinions. A minor character, she reflects a kind of small town person, but she also keeps her ears open for what is happening. She knows the pulse of the town.
We get to know the major characters well and want to know their fates. Their lives--and often their points of view--drive the plot. These major characters have both strengths and weaknesses and what we think of each of them determines how we feel about their fates. We must care about them, however, to stay involved in the story.