Antony and Octavius wait on the battlefield. Antony says that Brutus and Cassius are only attacking to make themselves look braver than they are. A messenger alerts them that the opposing army approaches. 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" (5.1.20).
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 and Cassius speak with Antony and Octavius before the battle. They taunt each other. Brutus and Cassius call Octavius young and inexperienced, and accuse Antony of being a social butterfly. Antony and Octavius call Brutus and Cassius hypocrites and traitors. Antony and Octavius exit.
At this point, the conflict is a power struggle nearly devoid of ideological disagreement. Brutus is no longer fighting for the good of Rome, but for self-preservation.
Brutus speaks apart with Lucillius. Cassius tells Messala that, though he never previously believed in omens, he was given a bad feeling by the fact that carrion birds now circle their army, when eagles once had done so. Brutus returns. He and Cassius say their goodbyes, because if they lose they will never see each other again. When asked by Cassius what he plans to do if they are defeated, Brutus says he does not believe in suicide.
This highlights the different philosophical schools to which Brutus and Cassius belong. Cassius, as an epicurean (those who believed the Gods were non-interventionist), had never believed in omens before. Brutus, as a stoic, does not believe in suicide, but that people should weather whatever life brings.