The presence of the crowd of commoners that fills the streets serves several dramatic purposes in Act III. Caesar has been assassinated in the Senate, and the Roman people are filled with shock and fear, surrounded by chaos. They become the audience for Brutus and Antony to address Caesar's death in separate orations. Their reactions to Brutus, who speaks first, bear out his belief that Caesar's death was justified and that the Romans will understand and accept it once circumstances are explained to them. Brutus's remarks are straightforward and logical; the people accept his words and embrace him as their leader.
Antony's subsequent speech, however, provides the dramatic climax of the play. Through effective persuasion and emotional appeals, he turns the crowd against the conspirators. No longer supporters of Brutus, they become instead an angry, violent mob that surges through the streets of Rome, killing and burning, and driving the conspirators out of the city. When the crowd becomes a mob, civil war has begun in Rome. Also, through the sudden change in the feelings of the Roman commoners, Shakespeare shows them to be unthinking people who are easily manipulated.