It’s no secret that side characters can be amazing in their own right. Great side characters feel like real people–even if the focus isn’t on them. They have lives that exist beyond the scope of the protagonist. When they seem to exist only to help or exacerbate the protagonist, they lack authenticity.
With that said, untamed side characters can water down a strong story, or worse, steal the story. While we don’t want our characters to be exact copies of each other (unless, of course, you’re writing a story about characters being exact copies), it can be helpful to examine the main character and his or her journey to bring balance, depth, and meaning to your cast. After all, side characters are also called supporting characters, which means they are meant to support the protagonist’s journey, not take away from it.