I’ve said this before, but the ultimate sin of any fiction is being boring.
Nobody wants to see a boring character on screen. Or on the page.
Think about the characters where people say, “they stole the show!” Those are the characters you want to write, because they resonate with readers.
Experience forms our choices, just like it will form your protagonists’. The supporting characters exist to help challenge them. I know I’m drilling this into your heads, but conflict and change is KEY.
Grab that list of your protagonists’ traits and habits. Write out the supporting characters as if to compare – and maybe contrast. Figure out where they were born, how long they’ve known the character, etc. Come up with your own beach episode – drop the characters on a random beach and decide how they would interact and react to each other.
Your main character isn’t the only one going on a journey. They can’t hold the giant pillar that is your story on their own. They need help, and those helpers can’t be flimsy pieces of paper.