It is important to note the chief similarity between main and side characters. That is, at any point, a main character might become a side character and vice versa. Their lives could stop. This is what helps them achieve personhood, this imagining them as people with control over their lives and not bound to some linear quest. Ultimately, this is what we want for our own lives. Besides entertainment, it is why we consume culture. To live vicariously through the thankless decisions of another that we might in some way find answers to our own dilemmas.
I am often reminded, in telling stories, of a scene in the Kubrick/Spielberg picture A.I.: Artificial Intelligence. David (played by Haley Joel Osmet) is telling Joe (Jude Law) of his plans to keep looking for the “Blue Fairy”, a distortion of the Pinocchio myth that the rest of the movie plays on extensively. He hopes to become a real boy so that his adoptive mother will love and accept him. It is a heart-wrenching tale that, for me, always hit too close to home (much like the original Pinocchio). In the scene, Joe implores him to instead give up his quest. He tells him that his mother’s love is not real and never could be real, no deeper than her love could be for a pet. He suggests David just stay with him and await the incipient destruction of humanity from the safety of the robot-friendly city. The story could have ended here, and not progressed. Another character, the main character, in a revision of the script could have happened upon them. Weighted with their own past and destination, the pain of David and his strange friendship with the pleasure-bot Joe would have been only a pale reflection to the main thrust of the story.
Thinking of character and plot this way not only helps flesh out the side characters, but develops the protagonist. What if they didn’t proceed? What reasons do they have not to? What palpable will or necessity drives them on if the consequences of settling down into history are not really that dire? This method also performs the function organically. You’re asking true and genuine questions of yourself and of your characters, as if you were them, rather than sitting down with a sheet of paper and filling in the blanks.
Motivation: etc. etc.
Worst fear: etc. etc.
Traumatic origin: etc. etc.
You see where I’m going with this. Perfectly fine books are written following the fill-in-the-blank formula. Many of them. But I know from my own work that things feel more alive, the narrative more present, if I’ve done as much as I can to create things organically. Formulas produce expected results. They can be relied upon. But isn’t it sometimes more entertaining, for the writer and the reader, to fuck something up and watch an explosion?