Instruments, tools, devices...
When a writer crafts their work they often regard characters as instruments in piece. A vessel for the audience to watch the narrative unfold. The creator will sculpt characters based on the demands placed upon them by the narrative.
As the writer begins sculpting they will define each character they will examine the influence of each character like a conductor chooses the best instrument for the piece. The characters begin as a list of roles, features and finally their influence. Are they a protagonist? Antagonist? The wise old man that will gift the protagonist with knowledge? Each character will have their roles defined and emphasized to blend smoothly within the piece.
Much as a conductor knows what instruments are available for a piece and how they will blend into the score a writer will often choose from the Jungian Archetypes. The psychologist Carl Jung defined the 5 potential archetypes as the psychological framework for universal prototype ideas. Every person, every character contains these psychological elements but the degree to which each archetype is emphasized will determine the reactions and ultimately the presented personality. These archetypes are described as the self, the shadow, anima, animus and persona.
The self is the filter, the means of controlling the psyche and facilitating individuation. While the shadow is the reverse of the Ego and the Anima/Animus are the male/female images within the psyche. Finally is the Persona, the outward appearance of the individual psyche to the world. In regards to characters, these archetypes can be visualized as the Child, the Hero, the Wise Old Man/Sage, Trickster and so forth.
While the characters are sculpted and chosen for the piece as a shopping list of qualities most creators will refrain from diving into the psychology of the characters. They begin with their archetypical roles and proceed to flesh out the characters through physical descriptions, selected commentary and influence. Instead of bringing the characters to life they are animated and it becomes reflected in the audience's reaction. Characters will struggle and bend to comply with the narrative at the behest of the writer. This is where characters will twist and perform actions that will assist the story but will not match their personas.
A number of creators will often sample personas that they experience in life and use them within the piece. Often these are the strongest characters. Sampling from the natural world, characters inherently have their own developed psychology and will ultimately feel more at home in a piece than the artificial characters sculpted from lists.
When a writer sculpts their characters for the piece they must make allowances for the personas, the psychology of the people in their narrative. When a character takes an action outside of their psychology it is akin to a sour note played from the orchestra. It will immediately jar the audience free of piece.