The main source of the play is Thomas North's translation of Plutarch's Lives.
Deviations from Plutarch
- Shakespeare makes Caesar's triumph take place on the day of Lupercalia (15 February) instead of six months earlier.
- For dramatic effect, he makes the Capitol the venue of Caesar's death rather than the Curia Pompeia (Curia of Pompey).
- Caesar's murder, the funeral, Antony's oration, the reading of the will and the arrival of Octavius all take place on the same day in the play. However, historically, the assassination took place on 15 March (The Ides of March), the will was published on 18 March, the funeral was on 20 March, and Octavius arrived only in May.
- Shakespeare makes the Triumvirs meet in Rome instead of near Bononia to avoid an additional locale.
- He combines the two Battles of Philippi although there was a 20-day interval between them.
- Shakespeare gives Caesar's last words as "Et tu, Brute? ("And you, Brutus?"). Plutarch and Suetonius each report that he said nothing, with Plutarch adding that he pulled his toga over his head when he saw Brutus among the conspirators, though Suetonius does record other reports that Caesar said in Greek "καὶ σὺ, τέκνον;" (Kai su, teknon?, "And you, child?") The Latin words Et tu, Brute?, however, were not devised by Shakespeare for this play since they are attributed to Caesar in earlier Elizabethan works and had become conventional by 1599.
Shakespeare deviated from these historical facts to curtail time and compress the facts so that the play could be staged more easily. The tragic force is condensed into a few scenes for heightened effect.