What is the power of speech in Julius Caesar?

The power of speech is seen most clearly in Act III when Brutus and Antony each speak at Caesar's funeral. Brutus speaks first, explaining very clearly and logically why he and the conspirators had assassinated Caesar. The crowd cheers him, and at the end of Brutus' speech, the citizens are ready to make Brutus a one-man ruler of Rome: "Let him be Caesar!" 

When Antony enters to speak, the citizens are quite satisfied with Caesar's assassination and even hostile to Antony: "'Twere best he speak no harm of Brutus here." Antony, however, begins his speech by creating a bond between members of the crowd and himself; they are his friends and countrymen and fellow Romans. From this beginning, Antony masterfully manipulates the crowd through verbal irony and strong emotional appeal until he turns them completely. By the end of Antony's speech, the crowd has gone wild, raging through the streets in search of the conspirators. They are so mindless and enraged that they deliberately kill an innocent man, Cinna the poet, solely because he has the same name as one of the conspirators.

Antony takes one phrase, "Brutus is an honorable man," and turns it into a sarcastic rebuke. Antony offers one example after another to show that Caesar was the honorable man, whereas Brutus is his cold-blooded killer:

When that the poor have cried, Caesar hath wept--

Ambition should be made of sterner stuff.

Yet Brutus says he was ambitious.

And Brutus is an honorable man.

Antony's strong emotional appeal also sways the crowd very effectively. He shows them his own suffering: "My heart is in the coffin there with Caesar, / And I must pause till it come back to me." He makes them believe that Caesar loved them, the proof being found in Caesar's will--which he cannot read to them because it would drive them mad to realize how much Caesar loved them now that he has been savagely stabbed by the conspirators' daggers.

Thus Antony controls the crowd, working them into a frenzy. At exactly the right moment, he dramatically pulls the cloak from Caesar's body, exposing his bloody wounds: "Look you here--/ Here is himself, marred, as you see, with traitors." Antony need say no more, but he does, suggesting that the proper response to Caesar's murder is "to rise and mutiny." The crowd, by this time, agrees completely, and the civil war in Rome has begun.