Secondary characters help add texture and depth to a book. They can be used for comic relief in an otherwise tense story. They’re fun to write. Harlan Coben’s Tell No One has great secondary characters – not full descriptions, just the essential elements for the story. It’s also got a nice plot with lots of twists.
You don’t need to tell your reader every bit of background for every secondary character. That’s going to slow the action and the reader may lose the storyline.
Consider this scenario for a romance:
Your protagonist is trying to get to the airport before her love interest leaves forever, and her car breaks down on a back road. When a stranger stops and offers her a ride, she’ll probably be hesitant about accepting. You do NOT need to give this character’s life story, or even what they ate for breakfast. Possible ways to handle their description:
Give the key descriptors that make your protagonist comfortable accepting the ride – there’s a baby in the back, or the driver is an awkward teen who is worried about getting into trouble for not going straight home if they give her a ride to the airport.
Show how the driver is somewhat sketchy, so she’s not sure she should take the ride, but getting to the airport is essential. The driver might share (in one sentence) an experience that made them go out of their way for her, probably when she’s getting out of the vehicle.
Let the driver be annoyingly chatty, going on about their own wedding, but have your protagonist tune it out to worry whether they’ll get there on time.