Shakespeare borrowed heavily from Plutarch's The Life of Julius Caesar for the information in his play. The following paragraph from Plutarch includes many of the "signs and apparitions" to be found in Shakespeare Julius Caesar.
Now, as for lights in the heavens, crashing sounds borne all about by night, and birds of omen coming down into the forum, it is perhaps not worth while to mention these precursors of so great an event; but Strabo the philosopher says that multitudes of men all on fire were seen rushing up, and a soldier's slave threw from his hand a copious flame and seemed to the spectators to be burning, but when the flame ceased the man was uninjured; he says, moreover, that when Caesar himself was sacrificing, the heart of the victim was not to be found, and the prodigy caused fear, since in the course of nature, certainly, an animal without a heart could not exist. . . . A certain seer warned Caesar to be on his guard against a great peril on the day of the month of March which the Romans call the Ides; and when the day had come and Caesar was on his way to the senate-house, he greeted the seer with a jest and said: "Well, the Ides of March are come," and the seer said to him softly: "Ay, they are come, but they are not gone."
While Plutarch took such supernatural matters seriously, Shakespeare did not. Rather he had a different purpose in using them. He used them as foreshadowing the assassination, building the audience's expectations up to the point at which they naturally assumed Caesar's death would be the high point of the play. Then Shakespeare deliberately disappointed them. He didn't want to feature a scene in which a bunch of men in togas pretended to stab an actor with wooden swords and daggers.
Shakespeare's plays emphasized the spoken word. He wanted Marc Antony's funeral oration to be the high point and the turning point. The assassination scene is anticlimactic. The stage directions are amazingly simple, considering all the previous buildup.
CASCA first, then the other Conspirators and BRUTUS stab CAESAR
After this there is nothing but confusion. The conspirators are all shouting contradictory orders, suggestions, questions. The audience feels disappointed at not receiving the long-anticipated emotional catharsis. Brutus eventually delivers a very formal and uninspiring speech to the assembled mob. And then finally Antony, with Brutus's permission, begins awkwardly and comically with the famous line
Friends, Romans, countrymen, lend me your ears
And gradually Marc Antony begins to weave a magic spell with what is probably the best thing Shakespeare ever wrote. He must have been inspired by the opportunity to recreate in English iambic pentameter the oration the real Antony gave in Latin some sixteen hundred years before. Antony has the Roman mob, and the English theater audience, in the palm of his hand. This is the cathartic moment they were led to expect. And the course of history is changed when Antony concludes with:
For I have neither wit, nor words, nor worth,
Action, nor utterance, nor the power of speech,
To stir men's blood: I only speak right on;
I tell you that which you yourselves do know;
Show you sweet Caesar's wounds, poor poor dumb mouths,
And bid them speak for me: but were I Brutus,
And Brutus Antony, there were an Antony
Would ruffle up your spirits and put a tongue
In every wound of Caesar that should move
The stones of Rome to rise and mutiny.