Do the minor characters in your story exude personality? Do they have small but noticeable eccentricities? Are they memorable in some or other way? If not, they need to be.
A bevvy of minor characters
In Dark City, detective Eddie Walenski, played by Colin Friels, is obsessed with drawing circular patterns on the walls of his dark prison-like room. He behaves like a man who has seen a terrible truth about existence and it has tipped him over into madness.
In Body Heat, D.A. Assistant, Ted Danson, is a minor character who pretends he is Fred Astaire, performing dance steps whenever he gets the chance. Odd but strangely captivating.
In The matrix, the Oracle is a minor character loaded with strong habits and mannerisms. She smokes like a chimney, drawing on her cigarette with excessive deliberateness, is obsessed with baking, and never answers a question directly.
In Down Periscope, writer David Ward creates a wonderful array of minor characters for his Lt. Dodge to engage with:
Nitro, the electrician is dumb, erratic, but very efficient at his job. It’s as if his I.Q. has dropped as a result of all the electrical shocks he’s received over the years. In order to have Lt. Dodge communicate with his superiors, Nitro has to turn himself into a conducting conduit each time!
Engineer Howard Elder is a sailor with many years of experience, which seems to have made him eccentric, if not downright wacky. He sports a filthy Hawaiian shirt and stubble. It’s as if Pearl Harbour has traumatized him so that time has stood still and he has never changed clothes.
Executive Officer “Mart” Pascoe is rigid and authoritative with a bad temper. His intimidation tactics are compensation for his diminutive stature. He repeats orders from Lt. Dodge by shouting them at the crew at the top of his voice.
Although the characters mentioned in the examples above are indeed minor in terms of the time and space they occupy in the story, each is made memorable through colorful mannerisms, eccentricities, or obsessions.
Minor characters need not be bland and flat, only serving the plot. Give them quirks and eccentricities to make them and your story more memorable.