If the main character is the heart of a story, I like to think of the secondary characters as the liver, spleen and kidneys. Sure, maybe you can lose a kidney and you'll be OK, but life sure would be a lot harder without these nifty organs hanging around. Why? Because they DO something all on their own, something completely independent of the heart.
That's one of the lessons I took away from my recent editing experience. We spent, to me, a surprising amount of time dealing with secondary characters. Not just in relation to the main character, but in relation to themselves. What did my secondary characters learn? What was their character arc? Did they have something to teach the main character, or something to learn from the main character? How do their stories support the theme?
Oftentimes, I see "stock" secondary characters. The needy spouse. The sarcastic best friend. The sadistic sidekick. This is OK, but I don't think it's enough if you want to write a really great book. It's not enough to have a character pop in simply to make a point about your main character, or just to give your main character an excuse to monologue or rage or whatever.
Because, c'mon, life just doesn't work like that. My own life is filled with "secondary characters," but the truth is, I'm a secondary character in THEIR lives. The only real one-sided conversations happen when one of the sides is being paid to shut up and listen, reflect, or emote (I'm thinking of psychiatrists and hookers, naturally).
So it goes with secondaries. They are living, breathing, actual people who are in my story because they, too, are on a journey. And even if their journey is ... well, secondary, it is important nonetheless, because I will not have a complete story unless the reader can feel the deeper currents at work. Unless the reader can sense that once these people walk off the page, they will walk into their own stories, this time as the star.
Easier said than done, right? Yep, at least for me. And that's why I'm in mad love with the Find function. Here I am, at the end of this story, and I'm reading for secondaries. So what I do is plug the character's name into Find and go through the book from the beginning, reading the entire book told only through their scenes, from their perspective. And along the way, I adjust, I delete and add, and explain, and eventually, if all goes well, I end up with a complete portrait of an important person -- not just a collection of quirks who happens to be convenient to my main character.