In respect of Character there are four things to be aimed at. First, and most important, it must be good. Now any speech or action that manifests moral purpose of any kind will be expressive of character: the character will be good if the purpose is good. This rule is relative to each class. Even a woman may be good, and also a slave; though the woman may be said to be an inferior being, and the slave quite worthless. The second thing to aim at is propriety. There is a type of manly valour; but valour in a woman, or unscrupulous cleverness, is inappropriate. Thirdly, character must be true to life: for this is a distinct thing from goodness and propriety, as here described. The fourth point is consistency: for though the subject of the imitation, who suggested the type, be inconsistent, still he must be consistently inconsistent. As an example of motiveless degradation of character, we have Menelaus in the Orestes: of character indecorous and inappropriate, the lament of Odysseus in the Scylla, and the speech of Melanippe: of inconsistency, the Iphigenia at Aulis,—for Iphigenia the suppliant in no way resembles her later self.
As in the structure of the plot, so too in the portraiture of character, the poet should always aim either at the necessary or the probable. Thus a person of a given character should speak or act in a given way, by the rule either of necessity or of probability; just as this event should follow that by necessary or probable sequence. It is therefore evident that the unravelling of the plot, no less than the complication, must arise out of the plot itself, it must not be brought about by the 'Deus ex Machina'—as in the Medea, or in the Return of the Greeks in the Iliad. The 'Deus ex Machina' should be employed only for events external to the drama,—for antecedent or subsequent events, which lie beyond the range of human knowledge, and which require to be reported or foretold; for to the gods we ascribe the power of seeing all things. Within the action there must be nothing irrational. If the irrational cannot be excluded, it should be outside the scope of the tragedy. Such is the irrational element in the Oedipus of Sophocles.
Again, since Tragedy is an imitation of persons who are above the common level, the example of good portrait-painters should be followed. They, while reproducing the distinctive form of the original, make a likeness which is true to life and yet more beautiful. So too the poet, in representing men who are irascible or indolent, or have other defects of character, should preserve the type and yet ennoble it. In this way Achilles is portrayed by Agathon and Homer.
These then are rules the poet should observe. Nor should he neglect those appeals to the senses, which, though not among the essentials, are the concomitants of poetry; for here too there is much room for error. But of this enough has been said in our published treatises.
Poetics by Aristotle