Shakespeare relied almost entirely on English translations of the Greek historian Plutarch's Life of Julius Caesar, Life of Antony, and Life of Brutus for all the information in his play. Plutarch, like his contemporaries of ancient times, had many superstitious beliefs which he incorporated into his Parallel Lives. Shakespeare used much of Plutarch's superstitious commentaries on historical events to capture the flavor of the times. Watching Shakespeare's Julius Caesar is a lot like traveling back in time to witness the events leading up to Caesar's assassination and the events that followed it. Here is a good example of Plutarch's writing from his Life of Julius Caesar:
So far, perhaps, these things may have happened of their own accord; the place, however, which was the scene of that struggle and murder, and in which the senate was then assembled, since it contained a statue of Pompey and had been dedicated by Pompey as an additional ornament to his theatre, made it wholly clear that it was the work of some heavenly power which was calling and guiding the action thither.
Plutarch believed that some supernatural power--perhaps the spirit of Pompey himself--was guiding the whole assassination conspiracy and wanted Caesar to die at the foot of Pompey's statue in retribution for the murder of Pompey, which was carried out by the Egyptians in order to curry favor with Caesar. Shakespeare, being a Christian, probably included such notions in his play because they were to be found in Plutarch, because they were intriguing, because provided material for good dialogue, and because they helped to capture the spirit of the age in which superstition was science. All the signs and wonders and prophecies, such as "Beware the Ides of March," were taken directly from Plutarch's Parallel Lives. Even Cassius, the most rational of all the characters in the play, became superstitious when the assassination of Caesar drew near. According to Plutarch:
Indeed, it is also said that Cassius, turning his eyes toward the statue of Pompey before the attack began, invoked it silently, although he was much addicted to the doctrines of Epicurus; but the crisis, as it would seem, when the dreadful attempt was now close at hand, replaced his former cool calculations with divinely inspired emotion.
Plutarch's description of the assassination of Julius Caesar takes up only a few paragraphs near the end of The Life of Julius Caesar. It makes stirring reading and must have been inspiring to Shakespeare.
And the pedestal was drenched with his blood, so that one might have thought that Pompey himself was presiding over this vengeance upon his enemy, who now lay prostrate at his feet, quivering from a multitude of wounds.
One impression the reader gets from Plutarch's description of the death of Caesar is that the conspirators must have been very frightened of the man, since it took so many of them to attack him.
For it is said that he received twenty-three [wounds]; and many of the conspirators were wounded by one another, as they struggled to plant all those blows in one body.
The death of Julius Caesar was a momentous event in Western history, so all the details that can be gathered seem to be worth preserving.