Actantial model
Greimas's model distributing a story's forces into six roles: subject, object, sender, receiver, helpers, opponents.
The actantial model, formalized by semiotician A. J. Greimas, reads narrative as a system of forces: a subject pursues an object; a sender motivates the quest, a receiver benefits from it; helpers assist, opponents obstruct. Actants are functions, not characters — one character can hold several roles; a storm or a desire can be an opponent.
For a writer it's an excellent diagnostic for soft plots: if the quest's object is vague, if no serious opponent obstructs the subject, the model shows it instantly. It also reveals fertile reversals — the day the helper becomes an opponent, the story thickens.
Example
The Grail quest: Perceval (subject), the Grail (object), God (sender), the kingdom (receiver).