This article, Layering Powerful Voice to Create Memorable Characters, while written back in 2005, remains evergreen.
"...how do you give several assorted characters, plus your main POV, powerful voices? Think of an onion. I'm serious! There are layers of skin to peel before you get to the good stuff in the middle of that onion. Building your character's voice uses the same technique; only this time you layer from the inside out. And the layers need to come in snippets – a fact here, a whisper or an overheard phone conversation there. Please, no dumping large chunks of informational text. Focus on blending in, little by little, the many physical and emotional layers that reveal each character's voice. Memorable characters are steeped in complexity and detail. Their layers are many and varied. Begin the layering process on the first page."And then Margot lists a dozen "how-to" suggestions, including
"Use comments from other characters to describe or praise or criticize your main POV character"
"Allow your main character some weakness he can either outgrow or overcome as the story progresses. No one is perfect. Kids know this - they sure aren't. Your reader wants to root for a character they can identify with."
It's an excellent piece, and well-worth reading.
I found this at Harold Underdown's Writing, Illustrating, and Publishing Children's Books: The Purple Crayon website. It's packed with resources and great information.
Illustrate and Write On,