On writing fantastic minor characters.
Most books have several characters; some of these characters are minor, which means they don’t show up very often, and/or they have little impact on the actual plot.
Some minor characters turn out to be nothing more than convenient scenery. Bob or Mary may be no more to your book than the fringe of cardboard grass that the set crew drags out for the pastoral scene.
To make your minor characters deep, give each of them a point of similarity to a major character, but make that similarity manifest in an opposite direction to the larger character.
Tobias hung back at the guard station; the robots were taking a long time today, and Tobias drew out his portable mech kit and began to twiddle with the controls on his artificial hand.
“Next,” the robot chanted. Tobias shuffled forward behind a fat Ovarsum pilot. Tobias twisted a delicate screwdriver under his knuckles, and adjusted the plate between his bone joints.
“Next,” the robot at the next station keened. “You, come over here!” the robot snapped. Tobias looked up; the line next to his had evaporated. He put his tool pouch between his teeth, and crossed to the robot, drawing his passage chip from his belt.
The robot took the chip from Tobias, and ate it. Tobias listened to the whir and churtle of the robot’s internal systems. He tightened the knuckle plates on his right hand, and put away his mech kit, flexing his fist open and shut.
The robot spit out Tobias’s passage chip, and threw it at Tobias, who caught it just before it fell to the floor. Tobias looked up at the robot, who regarded him dispassionately. Tobias noticed for the first time that the robot had a dysfunctional right hand; the knuckles sparked and emitted a puff of unpleasant white smoke.
The robot slammed the knuckles of its right hand against the window of the guard station, while staring straight at Tobias. The sparks in the robot’s hand died down, and the smoked lessened.
“Next,” the robot said.
Tobias began to adjust his artificial hand as he waited in line for the guard station to open up. The lines were long today, and the robots seemed very slow. Tobias wiggled a tool under the plates between his joints, and adjusted the tightness there.
“Next,” the robot said. Tobias balanced his tool kit on his knee, and shuffled forwards behind an Ovarsum pilot.
“Hey, you!” the robot in the next station over barked. “Come over here!”
Tobias looked up; he had not noticed that the line next to his had miraculously evaporated. He put his tools between his teeth, and got out his passage chip, which he handed to the robot.
The robot ate the passage chip, and spat it out.
“Next!” the robot barked, thrusting the passage chip at Tobias, who took it with his free hand.
Remember that every detail of your story is important to the reader; each minor character can build and reiterate the importance, nuance, and individuality of your main characters. Mirror and diversify to achieve deep emotional and rhetorical impact with your minor characters.