In Act 3, after the assassination, the protagonist who drives the rest of the action appears. Who is this person? What does he want? How were we prepared for his appearance in Julius Caesar?
Mark Antony emerges as a main driver of the play after Julius Caesar's assassination. Unlike Brutus, he is a ruthless man who truly believes the end justifies the means, no matter how brutal the means. As a gifted orator, he uses his speech at Julius Caesar's funeral to undermine Brutus, saying over and over "Brutus is an honorable man" in a way that makes mockery of Brutus's honor. Then he says, in his soliloquy after the funeral oration:
Now let it work. Mischief thou art afoot. Take thou what course thou wilt.
Antony is an older, ruthless man who wants power. His persuasive abilities earn him that power, as the crowd turns on Brutus and a civil war begins. Antony uses other people as tools and is willing to throw friends and family under the proverbial bus to achieve his goals.
Before his famous speech in Act III, we witness the loyalty Antony shows to Caesar in Act I, then earlier in Act III, we see how he falsely but persuasively addresses Brutus so as to be allowed to speak at Caesar's funeral. We are further prepared for Antony's betrayal when Cassius says, as an "aside," that Brutus has made a mistake in letting Mark Antony speak:
You know not what you do: do not consent
That Antony speak at his funeral.
When the stage is empty, Antony's soliloquy predicts the war and strife that will come. Caesar's revenge, he says, will "come hot from hell." It will be so bad that "Mothers shall but smile" when their infants are "quartered."
Yet Antony does not flinch from this.
The character who comes to the forefront in Act Three is Marc Antony. After the conspirators leave the scene of the crime, Marc Antony plots his revenge and then carries it out through the rest of Act Three. He uses his ability to command the language and his ability to persuade the commoners to get revenge on the conspirators, specifically Brutus and Cassius. Marc Antony is able to induce the conspirators to action so he doesn't have to be guilty of an unfavorable action.
He are prepared for Marc Antony and his tactics by his brief appearance in Act Once, scene two. He has few lines, but his loyalty to Caesar is evident: "When Caesar says do this, it is performed."
However, I would argue that Marc Antony is not the protagonist of this play. The play centers around Brutus and his decisions and consequences thereof. Brutus desires are better Rome, and figures that Rome will be better under the rule of himself or others, not Julius Caesar. Marc Antony is what stands in Brutus' way of establishing a new Roman republic. However, an argument can be made either way.