As explained in eNotes' "Introduction" in the Study Guide:
Probably written in 1599, Julius Caesar was the earliest of Shakespeare's three Roman history plays. Like Antony and Cleopatra and Coriolanus, Julius Caesar is a dramatization of actual events, Shakespeare drawing upon the ancient Roman historian Plutarch's Lives of Caesar. Brutus, and Mark Antony as the primary source of the play's plot and characters.
Plutarch attaches great importance to omens and the supernatural. According to Salem's Encyclopedia of the Ancient World:
His most famous work, the Bioi paralleloi (c. 105-115 c.e.; Parallel Lives, 1579), is organized into pairs of biographies of statesmen and military leaders, each pair consisting of the life of a Greek and that of his Roman counterpart (for example, Alexander the Great and Julius Caesar).
Plutarch's writings are full of references to the Greek and Roman gods and to omens, prophecies, and supernatural phenomena.
Shakespeare, however, could not have shared Plutarch's beliefs. He used ghosts, omens, and other supernatural material for different purposes in his play Julius Caesar. For one thing, they evoke the feeling of the time when everyone, including Julius Caesar himself, shared such superstitions. They help to carry the audience back into the remote past. They seem to be witnessing great historical events involving such famous men as Caesar, Antony and Brutus.
Shakespeare also used the supernatural to solve his biggest plot problem. Julius Caesar dies in the third act. Many critics have suggested that Brutus is actually a much more important character and ask why the play should not be named after him. Shakespeare used Plutarch to suggest that Caesar's body may have been killed but his powerful spirit continued to direct events until the very end of the play, and thus he was justified in naming it after Julius Caesar.
However, the great guardian-genius of the man, whose help he had enjoyed through life, followed upon him even after death as an avenger of his murder, driving and tracking down his slayers over land and sea until not one of them was left, but even those who in any way soever either put hand to the deed or took part in the plot were punished.
Shakespeare relied heavily on this passage. Caesar did seem to have had a great guardian-genius "whose help he had enjoyed through life." He seemed invulnerable and invincible. Whatever he was determined to accomplish, he did accomplish despite adversities and appalling odds. His soldiers idolized him. They would follow him anywhere. His enemies must have been terrified of him, since it took so many of them to assassinate him.
Shakespeare also used Plutarch's frequent descriptions of supernatural phenomena to foreshadow the assassination which takes place in Act 3, Scene 1. Because of all the foreshadowing in the first two acts, as well as the additional foreshadowing in the first part of Act 3, Scene 1--including the following, borrowed directly from Plutarch:
[To the Soothsayer] The ides of March are come.
Ay, Caesar; but not gone.
--the audience is led to believe that Caesar's death will be the climax of the play and might indeed be the conclusion.
Shakespeare, however, wanted to make the assassination anticlimactic and actually disappointing to his audience, because he intended to use Marc Antony's great funeral oration as the most dramatic moment and the pivotal point in the play.
If I may add to Mr. William Delaney's answer:
Famous Figures & Omens:
Julius Caesar (share this)
According to Suetonius (Lives of the Caesars, Julius 81) there were many warnings presented to Julius Caesar about his impending doom. He received bad omens such as the discovery of a table of brass that spoke of “a descendant of Iulus” being slain by his kinsmen, and a soothsayer named Spurinna observed various signs that something bad was going to happen to Caesar. There was such a fear that Caesar’s wife begged him to skip his final appearance before the senate prior to launching his next military campaign. Unfortunately for Caesar, he believed that avoiding the senate meeting would bring him shame and further problems.
In Shakespeare’s play, Julius Caesar, numerous omens foreboding the death of the eponymous lead character can be found. In Act 1, Scene 3, Casca informs Cicero about the omens that he has seen:
A common slave—you know him well by sight—
Held up his left hand, which did flame and burn
Like twenty torches join'd, and yet his hand,
Not sensible of fire, remain'd unscorch'd.
Besides—I ha' not since put up my sword—
Against the Capitol I met a lion,
Who glaz'd upon me, and went surly by,
Without annoying me: and there were drawn
Upon a heap a hundred ghastly women,
Transformed with their fear; who swore they saw
Men all in fire walk up and down the streets.
And yesterday the bird of night did sit
Even at noon-day upon the market-place,
Hooting and shrieking. When these prodigies
Do so conjointly meet, let not men say
"These are their reasons; they are natural";
For, I believe, they are portentous things
Unto the climate that they point upon.
(Shakespeare, Julius Caesar, Act 1, Scene 3, 15-32)
Cicero, however, doesn’t believe in superstitions, and remarks,
“But men may construe things after their fashion,
Clean from the purpose of the things themselves.”
(Shakespeare, Julius Caesar, Act 1, Scene 3, 34-35)
For students of English Literature, this ‘(mis)interpretation of omens’ becomes a major theme of the play. However, one may obtain other insights if one were to consider these omens from the viewpoint of a Classicist.
For a start, Shakespeare drew his inspiration for the play from Plutarch’s Parallel Lives, which is a collection of biographies of famous Greek and Roman figures. Indeed, much of the omens that were mentioned in the play were recorded by Plutarch himself. However, the meeting between Casca and Cicero is purely a fictional one, and invented by Shakespeare to highlight his point. After all, Cicero was regarded as the voice of reason.
Plutarch was not the only ancient writer who recorded the omens concerning Caesar’s death. In Suetonius’ The Twelve Caesars: The Life of Julius Caesar, it is mentioned that:
“A few months before, when the settlers assigned to the colony at Capua by the Julian Law were demolishing some tombs of great antiquity, to build country houses, and plied their work with the greater vigour because as they rummaged about they found a quantity of vases of ancient workmanship, there was discovered in a tomb, which was said to be that of Capys, the founder of Capua, a bronze tablet, inscribed with Greek words and characters to this purport: "Whenever the bones of Capys shall be moved, it will come to pass that a son of Ilium shall be slain at the hands of his kindred, and presently avenged at heavy cost to Italy.
(Suet., The Life of Julius Caesar, 81)
Suetonius even adds at the end, “And let no one think this tale a myth or a lie, for it is vouched for by Cornelius Balbus, an intimate friend of Caesar.” (Suet., The Life of Julius Caesar, 81). While Suetonius’ work is an interesting piece of writing due to its vivid portrayal of Roman emperors, it is also because of this feature that makes him a somewhat unreliable source. The fact that he needed to convince his readers of the authenticity of his claim should raise some doubts about how truthful he really is.
For a more serious piece of writing, one may consult Cassius Dio’s Roman History. Dio’s monumental work consisted of 80 books, although only Books 36-60 have largely survived the ravages of time. Fortunately, the death of Julius Caesar is found in Book 44. Like all good Romans, Dio took omens seriously, as seen in his recording of those that signalled Caesar’s impending death. According to Dio, the arms of Mars, at that time deposited in his house, according to ancient custom, by virtue of his position as high priest, made a great noise at night, and the doors of the chamber where he slept opened of their own accord. 3 Moreover, the sacrifices which he offered because of these occurrences were not at all favourable, and the birds he used in divination forbade him to leave the house. Indeed, to some the incident of his golden chair seemed ominous, at least after his murder; for the attendant, when Caesar delayed his coming, had carried it out of the senate, thinking that there now would be no need of it.
(Dio, Roman History, Book XLIV. 17)
However, Dio does seem to question at least the last omen, since he seems to hint that the incident became regarded as ominous in retrospect.
So, from the perspective of English Literature, we can see that Shakespeare’s presentation of the omens preceding the death of Julius Caesar was based on one ancient source. However, if one were to look at the issue from a Classical perspective, one is able to see that these signs were recorded by both sensational and serious writers. Thus, one may say that omens were part and parcel of Roman life. Many of the omens, such as Calpurnia’s dream, and the famous “Beware the Ides of March.”, have been left out from this article, so that you, the reader, could compare for yourself the various versions of the story that the ancient authors have left for us
- See more at: http://www.ancient-origins.net/myths-legends/famous-figures-omens-julius-caesar 001456#sthash.p9uJXFfN.dpuf