Directly after the assassination of Julius Caesar in Act 3, Scene 1, Mark Antony flees to his house to save his own life. But he realizes that he is not safe anywhere if the conspirators have decided to kill him too. He has no idea how many people they intend to slaughter, but he believes that all close friends of Caesar must be in extreme jeopardy, and he is the closest friend of all. It was he who offered Caesar the symbolic crown, showing that he intended to help his friend become the supreme monarch of the Roman Empire.
Antony decides that he must present himself before the triumphant conspirators and try to convince them that he is no threat to their proposed new political order. But he thinks it prudent to send a messenger first, and he instructs the Servant exactly how to act and what to say. He also tells the Servant to appeal directly to Brutus because he believes that Brutus is an honorable man, that he is the de facto leader of the conspiracy, and that he can be trusted to keep his word if he gives the Servant assurance of Antony's safety. Perhaps Antony senses that the co-leader of the conspirators, Cassius, wants to have him killed. At least he understands that Cassius would be much harder to fool than the idealistic, scholarly, unworldly Brutus.
The Servant approaches and, acting on Antony's instructions, kneels before Brutus and then falls prostrate--i.e. face down--while he delivers Antony's message. This is dramatically effective. What is happening is that Antony himself is symbolically falling prostrate, although he could not bring himself to do such a thing in person regardless of the danger he was in.
Thus, Brutus, did my master bid me kneel:
Thus did Mark Antony bid me fall down;
And, being prostrate, thus he bade me say:
Brutus is noble, wise, valiant, and honest;
Caesar was mighty, bold, royal, and loving:
Say I love Brutus, and I honour him;
Say I fear'd Caesar, honour'd him and loved him.
If Brutus will vouchsafe that Antony
May safely come to him, and be resolved
How Caesar hath deserved to lie in death,
Mark Antony shall not love Caesar dead
So well as Brutus living; but will follow
The fortunes and affairs of noble Brutus
Through the hazards of this untrod state
With all true faith. So says my master Antony.
Brutus has many noble qualities, but he repeatedly shows himself to be as much of an egotist as Julius Caesar. When Brutus addresses the mob he speaks mainly about himself and his honorable motives. He is flattered to have this messenger groveling before him, as Antony fully expected he would be, and also flattered to be addressed as the sole leader and sole decision-maker of his faction. Brutus is easily persuaded to guarantee Antony's safety. His response is:
Thy master is a wise and valiant Roman;
I never thought him worse.
Tell him, so please him come unto this place,
He shall be satisfied; and, by my honour,
Antony does not ask to be allowed to speak at Caesar's funeral until he appears in person and shakes hands with all the conspirators. He fully intends to try to turn the Roman people against the men who murdered his friend, but he has to proceed with extreme caution. Cassius wanted to have him killed, but Brutus overruled him. Cassius advises Brutus that it could be disastrous to let Antony address the assembled mob, but once again Brutus overrules him. Cassius wanted Brutus to be a figurehead, but he is learning that Brutus is impossible to control now that he has tasted political power.