Shakespeare's transformation of Plutarch's account has two main features which are in some ways opposite dramaturgic tendencies: the first is expansion, and the second is compression, or a telescoping of the action.
As in all of his plays, in Julius Caesar Shakespeare gives us a poetic treatment of his subject. Characters are permitted to expatiate at length on their situations, almost to the point of verbosity, but with such beautiful language that the characters themselves are ennobled. Plutarch and other historians tell us that omens accompanied the time of Caesar's death. Shakespeare hypothesizes lengthy conversations about these omens between Caesar and Calphurnia. In this, and in the scene of the assassination, we are shown a depth in Caesar's character which is hardly evident in Plutarch. The conspirators, as well, after they have killed Caesar, repeatedly and almost obsessively declare the nobility of their act and the ideological underpinnings of it: it was done for "freedom, liberty and enfranchisement." When they then rhetorically ask, "how many times will this our noble scene be played again in states unborn and accents yet unknown?" this is, of course, self-referential on Shakespeare's part. The essence of his take on the assassination is that of an analogy between Roman and specifically English values and English liberty.
The second feature of his transformed history is compression, the quickening of the pace of the events. In Plutarch's account, the uprising by the people against the conspirators did not occur as quickly and dramatically as in Shakespeare's depiction of it. The conspirators did, in fact, march arm in arm to the forum and make a long speech to the public to justify the killing of Caesar. But it was a number of days before they realized public sentiment against them was growing and that they needed to get out of town. In Shakespeare, all of this is telescoped into a single scene. The crowd applauds Brutus, but their approbation lasts only about five minutes once Antony makes his speech. Though Shakespeare's version takes liberties with history, it does so in the service of both drama and poetry and so ennobles Caesar himself, Antony, and the conspirators, showing that both sides of the conflict have their positive and negative traits. It's for us as readers to decide where our sympathies ultimately lie.