by Stu Leventhal
Your main character cannot deliver a well-rounded story all by him or herself. No one likes a braggart or even believes them so even when your main character is the voice telling the tale you need others to collaborate and add details.
Sure, supporting characters are important to your story but they should be there to help tell your reader more about your main character. They are props for your main character to play off of in scenes. Supporting characters can steal the show but only if it sets your main character up for acting a certain way later. Remember whose tale this is!
By the way you should also think of setting as another character that influences how your main character acts and develops.