In writing Julius Caesar, Shakespeare borrowed from two Classical biographies of important Roman and Greek figures, dramatizing the action and developing the historical figures into emotionally resonant characters. The most influential work was Plutarch’s Parallel Lives, which pairs biographical sketches of prominent Romans and Greeks. Parallel Lives contains many lines that are quite similar in content to Shakespeare’s version. For instance, in Plutarch, Portia attempts to get Brutus to confide in her, saying: “Brutus, I am Cato’s daughter, and I was brought into thy house not like a mere concubine, to share they bed and board merely, but to be a partner in thy joys, and a partner in thy troubles.” In the play, Shakespeare keeps the content of the source material but changes the order and wording to fit the proper meter: “Am I yourself/But (as it were) in sort or limitation,/ To keep with you at meals, comfort your bed,/And talk to you sometimes? Dwell I but in the suburbs/ Of your good pleasure? If it be no more,/Portia is Brutus’ harlot, not his wife” (II.i).

In several instances, Shakespeare dramatizes action that Plutarch summarized, turning exposition into character-revealing dialogue. For example, in Act I, scene ii, Caesar delivers an aside to Mark Antony regarding his distrust of Cassius. In the original text, Plutarch writes that “[Caesar] said it was not the fat and long-haired fellows that troubled him, but those pale and lean ones; meaning Brutus and Cassius.” In the play, Shakespeare offers a crucial edit: “Let me have men about me that are fat,/ sleek-headed men and such as sleep o’nights. Yond Cassius has a lean and hungry look,/…Such men are dangerous” (I.ii).Here, Shakespeare omits mention of Brutus, and limits the “danger” to Cassius. By emphasizing the intimacy between Brutus and Caesar, Shakespeare amplifies Brutus’ betrayal, adding weight to Caesar’s expression of surprise as he is stabbed in Act III (“Et tu, Brute?”). This makes the scene all the more powerful and poignant and casts a moral “doubt” on Brutus’ “noble” act to preserve the Republic.

Another important example of the way Shakespeare dramatizes Plutarch’s text comes at Caesar’s funeral. In Plutarch, Antony and Brutus’s speeches are merely summarized: “When the multitude was assembled there, Brutus made a speech calculated to win the people and fitting the occasion,” and “Antony pronounced the customary eulogy, and when he saw the multitude were moved by his words, changed his tone to one of compassion, and taking the robe of Caesar, all bloody as it was, unfolded it to view.” Shakespeare fleshes out the scene, adding depth and personality to his characters. In the original text Antony’s speech is portrayed as improvisatory, adapting suddenly in tone and sentiment to meet the fickle reactions of the public. Shakespeare, however, depicts Antony’s speech as purposefully subversive; the use of blank verse as opposed to Brutus’ prose, and the sarcastic repetition of “But Brutus is an honorable man” all give the impression of a deliberate attempt to undermine the conspirators.

The most famous quote of the play, “Et tu, Brute?,” spoken by Caesar right before he is stabbed by his beloved Brutus, marks another notable departure from the source text. The line, Latin for “you, too, Brutus?,” is lifted from Suetonius’s The Twelve Caesars, which describes the first eleven emperors of Rome. In the Suetonius’s text, Caesar is said to have cried “You too, my child?” in Greek (“Kai su, teknon?”). While cultured Romans of the period spoke both languages, an Elizabethan audience would have only recognized some rudimentary Latin, thus explaining why Shakespeare switched from Greek to Latin. More importantly, Suetonius suggests that Brutus was rumored to have been the bastard son of Caesar: “Beyond all others, Caesar loved Servilia, the mother of Marcus Brutus,” Suetonius writes. Despite being well aware of this gossip (there is a clear allusion to “Brutus’ bastard hand” in Henry IV), Shakespeare does not openly acknowledge it in Julius Caesar. Any mention of patricide (a son killing his father) remains unspoken, and is only possibly implied when Antony refers to Brutus as “Caesar’s angel” during his funeral oration.