Hale compared the main character in one's story to the sun. That character should be three-dimensional, which we've all heard before and hopefully, dutifully, and painstakingly accomplish with rewriting. However, he said that not all characters should not be as fully-fleshed out as the MC or they can outshine the star of your story. Makes sense.
When I heard this, it hit home because recent feedback on one of my manuscripts pointed to that very problem. An editor complimented two of my secondary characters, their role in the story, their personalities and quirks. Missing? The love for my MC.
If your secondary characters are stealing the spotlight from your MC, ask yourself why. Is it because you're writing from your MC's eyes and you're forgetting those details that help define his/her personality? Or maybe your secondary characters want to tell the story instead. Whatever the reason, revision is in order.
As I revise, I'll be paying particular attention to my MC and how he interacts with those noisy, boisterous secondary characters. He'll need to take center stage again, clamp a hand over their mouths, and remind them to be a little less show-offy. As the writer, I'm just the person to help him to that.