Action (praxis) is a term used by Aristotle in his Poetics (Theory of Poetry and Fine Art) to describe the fundamental element of the plot of a tragedy.
Action must not be confused with physical activity, deeds or events. Action as Aristotle understands the term refers to the motivation which spawns particular activities or deeds. Action is the, often unacknowledged and non-rational, urge, desire or appetite that contributes to our understanding of character. S.H. Butcher who has been deservedly praised for his translation of Aristotle's Poetics reminds us that “The action that art seeks to reproduce is mainly an inward process, a psychical energy working outwards; deeds, incidents, events, situations, being included under it so far as these spring from an inward act of will or elicit some activity of thought or feeling.”
A plot without action is merely an elaborate storyline and character without action is simply role play. For Aristotle plot was the soul of tragedy with character holding second place but they are, in a sense, isomorphic; they develop together as action is portrayed. For example, Hamlet was prone to procrastination, self doubt and long periods of reflection but his mistaking the innocent Polonius for Claudius and killing him was an impetuous act that sealed his fate. Hamlet's somewhat erotic relationship with his mother and his intense desire to avenge his father’s death promotes the uncharacteristic impetuousness that marks the climax of the plot; the point of no return in this tragedy.
In contemporary theatre the playing of the action (see acting) is understood as externalizing the motivation of a character which is the means of showing the audience the underlying psychological desires governing the deeds that delineate the personality of the character and advance the plot. The famed Russian director/actor Konstantin Stanislavski identifies this action as subtext. The externalizing of the action/motivation of a tragic character is necessary to allow the audience the opportunity to empathize with him or her rather than merely sympathize with the situation that entangles them.
For Aristotle “…the plot, being an imitation of an action must imitate one action and that a whole, the structural union of the parts being such that, if any one of them is displaced or removed, the whole will be disjointed and disturbed.” The ‘one action’ Aristotle speaks of is the action of the central character; the motivation that propels the character through the play. This is commonly understood as the through line of the action and is described by Stanislavski as the super objective.