Initially, Jocasta attempts to ease Oedipus's mind by explaining to him that the prophecy regarding Laius's fate was not fulfilled because he was murdered by three strangers at a place where the three roads meet. It was prophesied that Laius's son would one day kill him. As a result of the earlier prophecy, Laius and Jocasta took precautions by fusing their son's ankles together and leaving him in the mountains to die as an infant.

Later on, a messenger arrives from Corinth and informs Jocasta that Oedipus's father, Polybus, has died from illness, which means that the prophecy did not come true. Jocasta is once again reassured because she is under the belief that Polybus is Oedipus's biological father. However, the messenger then informs Oedipus that he is not Polybus's biological son and says that Oedipus was discovered as a child in the mountains by a shepherd. The messenger recalls a shepherd saving Oedipus as an infant and then bringing him to Polybus, who raised him as his son. It is at this moment that Jocasta begins to suspect that Oedipus is her son and that she is guilty of committing incest. Jocasta begs Oedipus to end his investigation and flees the scene in terror when Oedipus sends for the shepherd.

Jocasta pretty much gives the game away when the messenger arrives to tell her and Oedipus the story of how the infant Oedipus came to arrive at the house of Polybus and Merope.

Oedipus wanted the shepherd to corroborate this story, but the shepherd—literally—took to the hills as soon as Oedipus arrived in town, as he didn't want to tell him what happened. For obvious reasons, Jocasta feels exactly the same way. The last thing she wants is for the shepherd to say his piece, thus encouraging Oedipus to investigate his origins further.

It's at this point in the drama that we realize that the penny's finally dropped for Jocasta. Why else would she explicitly dissuade Oedipus from getting to the bottom of things? Why else would she say something like this:

Why ask who he means? Don't pay any attention to him. Don't even think about what he had said—it makes no sense.

Jocasta's never said such a thing before. Perhaps she'd previously had her suspicions, but now we know for sure that she's aware of the terrible truth, and she'll do anything she can to prevent it from getting out.

Jocasta first becomes uncomfortable when Oedipus tells her of his dilemma and the oracle's prophecy. However, it is when the messenger arrives with news of the death of Polybus that the pieces really begin to come together for her. After the messenger reveals the Polybus and Merope were not in fact Oedipus's birth parents, her initial fear of the oracle's prediction sinks in as reality. As the messenger tells them of the story of how Oedipus came to live with Polybus and Merope, he tells Jocasta that she can identify the servant who had...