“What is character but the determination of incident? What is incident but the illustration of character?” HENRY JAMES
As David Morrell reminds us in his book LESSONS FROM A LIFETIME OF WRITING, plot and character are intimately related. Every character that comes on scene has interaction with your main character and will establish a relationship with that main character. Your protagonist will interact with each of those minor characters in some way, and their actions and dialogue move the plot along. Is that relationship from the past or a new one? (Be sure to add Morrell’s book to your writing reference library.)
According to E. M. Forster, main characters are multidimensional. They surprise us, they are complex, and they are difficult to describe succinctly. They are defined by who they are.
The iceberg theory is a style of writing coined by American writer Ernest Hemingway. “The dignity of the movement of an iceberg is due to only one-eighth of it being above the water”. Same is true for characters and their stories. What is obvious to the reader implies a much larger truth and depth. That is why the majority of your character profile will never appear in your story, but you know your characters intimately. As one workshop instructor explained, the writer should know what is in the main character’s closet.
What does your character want, and what obstacles can you throw in their way to prevent them from achieving that goal?