Secondary characters are often glossed over in popular fiction. They’re in the story to further the plot, and can sometimes devolve into broad stereotypes or cardboard cut-outs while the protagonist (and maybe love interest) dominate the story.
You can avoid the problem if you think of the secondary characters as “supporting actors” like the ones who get the interesting awards at the Oscars. You know: the ones that have you saying, “I’ve never heard of her, but she looks brilliant in that clip. I’ll have to check out the film”
When I was an actor, I usually auditioned for those “secondary” characters. I found them more interesting and challenging. I went for Vera Charles instead of Auntie Mame, Mrs. Gibbs instead of Emily Gibbs, The Wicked Witch instead of the Good Fairy. I used to say I specialized in witches, bitches, and moms.
There’s an old joke in the theater about the annoying minor player who constantly asks the director, “What’s my motivation?” But that is exactly the question you should be asking about your secondary characters.
Why does that hot mechanic who keeps the heroine’s ancient VW Rabbit alive happen to be driving by her house at midnight when she’s being attacked by evildoers? Why does the sassy best friend always appear when she’s called? Doesn’t she have a family? A life? And what about those evildoers? Why are they doing evil, anyway?