Octavius, Antony, and their army are waiting on the battlefield. Antony thinks that Brutus and Cassius are attacking them in order to make themselves look braver than they are. A messenger alerts them that the opposing army is approaching. Antony gives Octavius an order about how to advance his troops, which Octavius disputes. When Antony asks why Octavius is arguing with him, he replies, "I do not cross you, but I will do so.”
Just as omens can have ambiguous meanings for the characters, but definite ones for the audience, characters' speech can work the same way. Octavius's remark has a double meaning, since he will eventually betray Antony and rule alone as Augustus Caesar.
Brutus’s and Cassius’s army meets Antony’s and Octavius’s army on the battlefield, and they exchange taunts. Brutus and Cassius mock Antony as an untested soldier, a bee who is all “buzz” and no sting. Antony and Octavius call Brutus and Cassius hypocrites and traitors. Antony and Octavius exit in disgust.
At this point, the conflict between the two sides is a power struggle, not primarily a matter of ideological disagreement. In contrast to his earlier high-minded principles, Brutus seems to be no longer fighting for the good of Rome, but mainly for self-preservation.
As Brutus steps aside with Lucilius, Cassius talks with Messala. Cassius says that, although he’s an Epicurean and previously gave little credit to omens, he has changed his mind—carrion birds have been flying above the army and looking down at them as if they’re prey. When Brutus returns, Cassius says that, although he’s hopeful about the battle, in the event that they lose, this will be the last time he and Brutus speak together. Brutus says that if they lose, his Stoic outlook will prevent him from committing suicide. However, neither will he be content to be led as a defeated captive through the streets of Rome. The two say farewell to each another.
This exchange highlights the different philosophical schools to which Brutus and Cassius belong. Cassius, as an adherent of Epicureanism (which rejected the idea of divine intervention in human events), never previously believed in omens. However, the events of this momentous day make him rethink his stance. Brutus, as a Stoic, does not believe in suicide, but holds that people should calmly accept whatever life brings. To some extent, his remark here belies the manner of his eventual death.