One of the most gratifying notes I ever got from my editor was on my upcoming novel FML. When speaking about a character (I won't name her so as not to spoil it), my editor said something like: At first I totally hated this girl, then couldn't stand how toxic she was, then I just wanted to hug her!
I'm paraphrasing because I'm writing this from a location not my home, in which I don't have access to my editorial letter. I was so thrilled because this particular character was an afterthought. When I began writing her, I threw her in as a way to help turn my plot on its head. I tossed her into another scene to help move the plot along. Later, I realized how cool a character she was, but in a book already stuffed full of characters, I didn't want to add another major. So, even though she only appears in a couple of scenes, I did my best to give her a gratifying arc.
When my editor mentioned her reaction to the character, it was a relief because it meant I'd managed (somewhat through luck) to elevate that minor character's journey into something my editor cared about.
And I think that's what helps good books really connect with readers. I liked Harry, Ron, and Hermione as characters, but it was Neville I genuinely rooted for. Luna I connected to. Their story arcs were almost as important to me as Harry's was.
There's a great line in FML that goes something like, "You may be the hero in your story, Simon, but in everyone else's, you're just a minor character who never achieves enough depth for anyone to care about."
Don't let your minor characters fall to the wayside. Nurture them, make them real. Because heroes might be larger than life, but secondary characters are who life is all about.