In the very brief Scene 3 in Act II, Artemidorus, a minor character, is shown reading a letter he has composed and intends to present to Caesar as he passes by on his way to the Capitol. The letter contains a warning that there is an assassination plot against him and he is in grave danger at this very moment. Artemidor names the names of all the conspirators beginning with:

"Caesar, beware of Brutus. Take heed of Cassius."

Artemidorus reads his letter aloud and then concludes this scene by saying:

If thou read this, O Caesar, thou mayst live. 
If not, the fates with traitors do contrive.

Shakespeare's purpose in introducing Artemidorus and his warning letter is to build up the tension in the scenes leading to the actual assassination. In Act 3, Scene 1, the audience sees Artemidorus trying urgently to present his letter to Julius Caesar. Decius, who is one of the conspirators, interferes, probably because he suspects that Artemidorus' letter is a warning of what is about to happen. Decius says:

Trebonius doth desire you to o'erread
At your best leisure this his humble suit.

Artemidorus is insistent:

O Caesar, read mine first, for mine's a suit
That touches Caesar nearer. Read it, great Caesar.

But Caesar, always playing the modest public servant, replies:

What touches us ourself shall be last served.

So Artemidorus is thwarted and the assassination plot draws ever nearer to its long-anticipated accomplishment. The conspirators all crowd around Caesar under the pretense of presenting a suit. They completely shut out all the others and cut off any possibility for Caesar to escape. The audience is in suspense, expecting to see a reenactment of a great moment in history.

But Shakespeare has a trick up his sleeve. He knows that violent action on a stage always looks faked, and this would be especially hard to stage with so many actors involved. Shakespeare's forte is his poetry spoken by his characters. He wants to make Antony's funeral oration the high point of the play. His description of Caesar's assassination suggests that the event is short and simple.

They stab Caesar, Casca first, Brutus last.

CAESAR
Et tu, Brute? Then fall Caesar.

He dies.

He dies. Two words describe his death! No doubt it is intended to be over with very quickly on the stage. This will seem disappointing and anticlimactic to the audience, a real letdown after the buildup that has been growing more and more intense since the beginning of the play. The audience would be expecting an emotional catharsis--but they will not get it until Mark Antony mounts the pulpit and begins with

Friends, Romans, Countrymen...

Then the audience will hear the most inspired, the most seemingly inexhaustible eloquent blank verse that Shakespeare ever wrote. He must have been delighted with the prospect of being able to recreate in English iambic pentameter a speech that changed the course of history some sixteen centuries before. 

Shakespeare borrowed the incident involving Artemidorus, a teacher of Greek logic, from Plutarch's Lives of the Noble Grecians and Romans in an English translation by Thomas North.

O Caesar,