Just as your primary characters—protagonist/antagonist, hero/heroine, villain—need to be well developed, three dimensional characters, so must all secondary characters be real people the readers can connect with. However, it is important to not let the secondary characters take over.
• Example: The movie Robin Hood: Prince of Thieves, starring Kevin Costner as Robin Hood. Allen Rickman, the actor playing the Sheriff of Nottingham, did such a tremendous job of bringing his character to life that many reviewers claimed he upstaged Robin Hood. They said his character was far more interesting, lively, and three dimensional than the main character of Robin of Locksley. As a whole, this movie was a box-office hit, but the reviewers only gave it two stars.
In a fiction book, the same thing applies. You cannot make secondary characters more interesting than the main character about whom the book was written.
Now in the first sentence of this section, I said all secondary characters must be just as real and likeable as the main characters, but their story must be a smaller part of the whole book. This leaves you with the huge challenge of balancing just the right amount of importance each character deserves in the book.
Often, in Christian novella collections the first book will contain secondary characters who become the main characters in the second story. The second story then contains a secondary character who becomes the main character in the third, and so on.
In each of these novellas the author is careful not to let the secondary characters play a part so big that the reader might wonder who the real heroine/hero is. This technique must be applied to full-length novels too.
If you find that your secondary character is getting more attention and is more interesting than the main one, don’t delay in changing it. However, it’s possible that if the secondary character is simply too important to ignore, your book may need to become two, each with different main characters. Either way, you need to rein in the secondary character and not let him or her overshadow the main character.
You will still want to fill out a character chart for secondary characters in order to make them three-dimensional and interesting. Just remember to focus on the main characters and their relationship with the plot.
Put those secondary characters in their place at all times!