Knowing that Elizabethans were familiar with the history of Caesar's assassination, Shakespeare wrote his historical play, Julius Caesar, placing into blank verse much of Sir Thomas North’s "wonderfully idiomatic" translation of Plutarch’s Bioi paralleloi (c. 105-115; Parallel Lives, 1579). In this translation, there is this particular passage which describes the arrival of Julius Caesar at the Senate:
As Caesar entered, the senate rose in his honour, but as soon as he was seated the conspirators surrounded him in a body, putting forward Tullius Cimber of their number with a plea in behalf of his brother, who was in exile. The others all joined in his plea, and clasping Caesar's hands, kissed his breast and his head.
As they surrounded Caesar, the conspirators moved in, taking advantage of Caesar's predicament and began to stab him.
In Shakespeare's play, Act III, Scene 1, just as in Plutarch, Popilius Laenas approaches Caesar and speaks to him, alarming Cassius, who fears that Caesar is being advised of the conspirators. At this point Brutus tells Cinna, "They're speaking to him. Go up there and second his petition" (ll.32-33). Cinna responds, "Casca, you are the first that rears your hand" (l.34) and Casca is, then, the first to put a dagger into Caesar.
Not all the senators were in accord that Caesar should be assassinated. This is why Cassius persuaded Brutus to join the conspirators: they needed one of his honor and respected position to give their cause legitimacy with the Roman public.