A few days ago, I was talking on the phone with my five-year-old niece, "C1." When she asked what was new with me, I told her that I was making a blanket (the prototype of a crochet pattern I'm designing for my Etsy shop). Immediately "C1" asked, "Is it for 'C2' [her little sister] or me?"
Considering how many times I've made each of the girls a blanket, this was a totally understandable question. Especially from a five-year-old.
My sister-in-law, who was listening, and I both laughed. She said, "'C1,' not every blanket is for you."
This reminded me of a useful bit of writing wisdom:
Every character thinks the book is about them.
Sometimes authors write as if the protagonist is in charge of the story and the other characters just follow his/her lead and react to what s/he does. But every character should have their own goals, personalities, and ideas about how the world works. And at least some of those qualities should cause them to clash with the protagonist, even if both characters are "on the same side." Here are some examples:
Han Solo's greed vs. Luke Skywalker's selflessness (original Star Wars trilogy)
Spock's rationalism vs. Kirk's often-emotion-led thinking (Star Trek: The Original Series)
Peggy Carter's hard-won self-assurance vs. Captain America's lingering awkwardness and self-doubt (Captain America: The First Avenger)
Remember, even though you know the book is about the protagonist, your characters don't. Write them accordingly.
(Thanks to Thibault Dandré for sharing their work on Unsplash.)