In On Writing Stephen King says:
It's also important to remember that no one is "the bad guy" or "the best friend" or "the whore with a heart of gold" in real life; in real life we each of us regard ourselves as the main character, the protagonist, the big cheese; the camera is on us, baby. If you can bring this attitude into your fiction, you may not find it easier to create brilliant characters, but it will be harder for you to create the sort of one-dimensional dopes that populate so much pop fiction.
It's easy to put so much energy, so much thought, into creating your main characters that all those secondary characters wind up being cardboard cutouts. "The best friend" or "the wacky sidekick" or whatever else have a purpose to fulfill, in relation to the story and the main characters. The trick is to not write them as if that's all they're for. The trick is to write them as if there is an alternate universe of the fictional world you are creating and in that alternate universe, that secondary character is the main character. Some of the best, most fully realized novels I've ever read had secondary characters that were so interesting, I would gladly have read a book about their adventures.
One of the best examples of this I know is the Harry Potter world. While I understand why JK Rowling doesn’t want to spend the rest of her life writing Wizarding World stories, as a reader nothing would delight me more than reading the further adventures of some of the side characters, especially George Weasley and Neville Longbottom. In fact I loved the entire Weasley family and loved every minute they were on the page. As for Neville, his own journey through the course of the books was every bit as amazing, and occasionally heartbreaking, as Harry's. When Neville had his big moment in the Battle of Hogwarts I cheered so much I dropped the book. In the hands of a lesser storyteller, Neville would have been just the stereotypical nerdy picked-on kid, and a prop to reflect aspects of Harry's own journey. But instead we saw glimpses of a young boy on his own Hero's Journey, every bit as compelling as the main character.
Who are some of your favorite secondary characters that you'd love to read more about?