Sometimes to critique an idea in a story, you might employ a milepost character. This is a character who never changes though the main character’s perspective of him does. It’s a term coined by David Smith in an article for the Science Fiction and Fantasy Writers of America.

Suppose, for example, that you wanted to show how the idea of materialism is a bad concept. You might personify that idea through a milepost character, say a banker at a larger corporate financial institution. The banker could enjoy a life in which he wears the best clothing, drives the best cars, and owns the largest of houses.

At first, the focus character – who is a new employee at the bank – might admire the banker, who compliments him and is courteous to others. As the story evolves, however, the focus character may learn more about the banker, maybe catching the latter in a moment of criticizing the poor or turning down a house loan for a hard-working single mother. The focus character might chalk up the former to drink and the latter to “That’s business,” but it taints his perspective. Perhaps when the banker decides to foreclose on a long-time friend who’s missed two payments, the focus character’s views of what qualities are the best in humanity continues to transform. In this way, the reader along with the focus character begins to question the value of materialism.

In addition to critiquing an idea, the milepost character offers the obvious advantage of showing how the focus character is growing and developing. As if a scientific experiment, the reader now can use the milepost character as a basis of comparison with the focus character’s perspectives.

When creating a milepost character, avoid creating a flat stereotype. While the milepost character doesn’t grow, there ought to be good reasons for his motivations and actions (In the above example, for example, the banker might argue that he can’t help everyone or the institution would collapse, and that would be bad for the community overall, as the bank helps create businesses that employ people.). Rather than be a comic character (unless writing humor, of course), the milepost character can be somewhat tragic, a prisoner of his own inability to change or escape the trappings of the idea he personifies.