I want people to see my secondary characters and wonder what their life is like. Wonder what they do when they're not around the main character. Wonder what their favorite movies are and how they do their hair in the morning and what their dog's name is. I want all of my characters to seem authentic and real.
I know some authors who write scenes from their secondary characters POV - stuff that wouldn't end up in the book - just to understand their personality more and make them come alive. I think that idea is brilliant. The more time you spend in your characters world and the more details that come together, the better off the book will be.
And I'm not saying every single character should be full-fledged: the girl sitting behind your MC in history class, who has no relevance to the story, doesn't need a full chapter on her life. But if you give her a tiny detail, like the fact that she has a tattoo of a sea turtle on her neck, it makes them more dimensional and adds to the realism.
So next time you're writing and you throw in a character that's in your MC's ballet class, think about their own story. Who their parents are and what their hobbies are and where they work and what their favorite kind of candy is. Make them breathe. Your story will be better for it.