You may have the most fascinating, debonair, capable protagonist of all time, but he still can't carry the story on his own. He needs help. He needs supporting characters.
What would James Bond be without the people behind him? Who would Sherlock explain his theories to if Watson wasn't there? How effective would Big Brother’s Danielle have been if she hadn’t had the totally wacko Evel Dick to do her bidding?
But you can't just stick people into your story willy-nilly. They have to have purpose and importance.
You probably already have your protagonist. You know why he's there and what's he's going to do over the course of the plot. You know his or her arc and how it fits into the theme of the book. Now, who does he need to accomplish all you've set out for him? Who will mirror or contrast her? Who will take on the task of giving the reader information so that the main character doesn't end up lecturing or pontificating?
Think first of who would naturally be around your protagonist. A teacher needs students; a father has kids; a CEO oversees underlings; a produce clerk has managers and customers. Then think how these people can be used to show the status of the main character, how they can shine the light on him or her that illuminates the qualities you want the reader to see.
Subordinate characters do more than just reflect the importance of the protag, though. How do they convey the theme of your book? Keep in mind that they can help demonstrate the theme, while also representing different sides of that theme. No theme has just two sides. It's not all black or white. You know the theme of your book; now consider the different angles to that idea, the varying statements each secondary character could make about that theme. Once again, they are helping the protagonist carry the story. But, while they're doing it, they don't have to all be just like the main guy or clones of his beliefs and emotions.
You want variety in the make-up of your story people. They don't all look alike or sound alike. Each has a different background or upbringing. It's only logical that each would also have his or her own attitude and value system. Consider your theme or message. How would the protagonist see this theme? What would he think or do? How does it affect him? How does she change in relation to this theme over the course of the plot? Now, do that same thinking for each character. See it from each one's viewpoint and personality. Sure, some characters are similar to the protagonist -- we tend to be drawn to people who hold a value system akin to our own. Remember, though, opposites attract. But while we have the extremes of black and white in our lives, we also welcome all the *gray* in-between.
And, of course, secondary characters can give out information. Just make sure that information carries the story forward, rather than bringing it to a halt. You don't want your protagonist lecturing the reader on things he or she already knows, nor do you want the secondary characters to do that. But if you've got to get stuff across to the readers, sometimes it's better to have a side-kick do it.
You want secondary characters in your story, but they need to have purpose. They're not just there to add color or fill in the background. They can give so much more to your book. They can shine a light on the protagonist (and the antagonist, incidentally) that highlights the good qualities as well as the faults. They move the story forward. They have importance.
We all want to feel important. Even if we're only side kicks. Maybe especially if we're side kicks
1 day ago