In the religious plays Racine is fairly scrupulous in adhering to his Old Testament sources, taking care to put into the mouth of Joad (the Second Jehoiada) only those prophetic utterances that are to be found in the Bible. Nevertheless, he takes advantage of a verse in II Chronicles XXIV attributing the gift of prophecy to Joad's son Zacharie (Zechariah ben Jehoiada) in order to suppose that the father (whom the Bible does not describe as a prophet) likewise had prophetic powers. And thinking a child of seven – the age of Joas (Jehoash of Judah) in the Second Book of Kings – too young to have the part given him in Athalie, Racine makes him into a boy of nine or ten on the evidence of the Septuagint version of II Chronicles XXIII 1.

In the secular plays he takes far greater liberties. The frequently conflicting sources of Greek and Roman mythology enable him to fashion the plot he thinks suitable to his characters and, above all, to present the old stories in a modern light. Whereas Euripides, in his Iphigenia in Aulis, only averts the heroine's death by causing Artemis to spirit her away to Tauris, putting a hart in her place on the sacrificial altar, Racine, determined to avoid the miraculous, borrows from a minor Greek writer, the geographer Pausanias, the character of Ériphile. The disclosure that Iphigénie's treacherous rival was herself called Iphigeneia at birth and should be sacrificed in the heroine's place prevents a tragic outcome.

In creating Andromaque, Racine believes he must “[se] conformer à l'idée que nous avons maintenant de cette princesse”. Astyanax, whom Euripides describes (in The Trojan Women and the Andromache) as having been thrown from the walls of Troy and killed, and whose death is foreshadowed in book 24 of the Iliad, is made to survive the capture of Troy and the extinction of his dynasty. In another respect also Racine departs from the lines laid down by the Andromache, for whereas in the earlier play the heroine fears that the son she has had by Pyrrhus may suffer death if she refuses to marry the father, the later heroine fears for the life of a legitimate son. The reason for these changes in the Homeric and Euripidean traditions is obvious: if Andromaque had been Pyrrhus's mistress (as in Euripides), why should she refuse to marry him? Racine, like Homer, conceives her as sublimely faithful to Hector; yet the tension (III 8) between maternal love and a reluctance to marry Pyrrhus must (as in Euripides) be paramount. And so Astyanax is brought back to life.

Phèdre differs from Euripides's Hippolytus and Seneca the Younger's Phædra in the very important respect that, taking the character of Aricie from Virgil, Racine introduces the jealousy motive. Despite the fact that Hippolyte, “ce fils chaste et respectueux”, is indifferent to her, Phèdre will not consent to Œnone's suggesting to Thésée that the son has made improper advances to the stepmother – until (IV 5) she discovers that he has loved Aricie all along.

Racine observes the dramatic unities more closely than the Greek tragedians had done.

The philosopher Aristotle points out the ways in which tragedy differs from epic poetry.

“Tragedy”, he observes,

"generally tries to limit its action to a period of twenty-four hours, or not much exceeding that, whilst epic poetry is unlimited in point of time."

Writing 100 or 150 years after the great Attic tragedians and using their works as a basis for generalization, he does not insist that the action of a tragedy must be confined to a single revolution of the sun, or that it must take place in one locality. He merely says that this limitation was often practised by writers of tragedy, but he well knew that there were many plays in which no such limitation existed. For instance, Æschylus's Agamemnon compresses into about fifteen minutes a journey (from Troy to Argos) which must have taken several days.