CriticaLink | Aristotle: Poetics | Guide to Book XV
Poets should pursue four goals in constructing their characters, according to Aristotle.
For Aristotle, the most important aspect of a character is goodness, which seems to be linked to some sense of the character's intentionality. The sharp hierarchization of ancient Greek culture is evident in Aristotle's remarks about the possibility of a woman or a slave to exhibit goodness. Even though he concedes that this is possible, the statement betrays the subaltern status of both.
The behavior of characters must be suitable for their social rank. Here again, the stratification of the society of Aristotle's time is quite clear.
Characters must be believable. Attributes assigned to characters must conform to what would be expected from the same kinds of persons in real life.
The kinds of behaviors assigned to a character must not change suddenly and inexplicably; if a character is meant to be represented as indecisive, erratic, or otherwise inconsistent, this inconsistency must be consistently portrayed.
Deus ex machina
Aristotle is concerned with preserving the identification of the audience with the actions depicted in the tragedy. Any of us who have read novels or seen plays or films in which something doesn't "ring true" can understand how flaws in plotting and characterization can interfere with our capacity to get caught up in the story. Both character and plot must be consistent and develop in ways that conform to the laws of probability, Aristotle argues.
He restricts the famous theatrical device of the deus ex machina, the "god from the machine," from taking part in the plot itself. The gods should not intervene to resolve an impossibly complicated plot. They can, however, appear to cast judgment on the characters once the plot has come to its necessary conclusion.