Well, for one, I try to write the characters in a rounded way. Their dialog and reactions aren't clearly attached to strings that I'm moving diabolically in the background.
This kind of ineffective technique is present in, I think, Uncle Tom's Cabin, when the black characters and white characters both act as archetypes that are talking for the writer's skewed view of race relations in America, rather than being individuals. This is partly why the novel got so much flack. The white woman who wrote it knew nothing of being a struggling black in America, and used her whites simply to preach a message of tolerance that was, despite her best efforts, still racist.
None of the characters, not even secondary ones (IMO), are there to preach your message as a writer, or as an artificial plot element. They fulfill the role of obstacle or mood elevator or whatever else, but must be shown also as organic, complex humans. Each encounter with a person, however brief and undeveloped in drama, should feel like an encounter with an actual person. Mannerisms, dialog, free will, etc.
Sorry I wish I could explain it better but this is briefly my approach.