William Shakespeare's historical tragedy, The Tragedy of Julius Caesar, was first performed in September of 1599. Julius Caesar might well have been the first of Shakespeare's plays to be performed in the newly-constructed Globe Theatre.
At the opening of the play, the people of Rome are in the streets to welcome Caesar back to Rome and to celebrate his victory over Pompey. A Roman tribune, Publius Servilius Casca Longus, known in the play simply as Casca, gets caught up in Caesar's procession through the streets. Casca is present when Caesar refuses the crown as king of the Roman Republic, which is offered to him by Marc Antony to the shouts and cheers of the Roman people.
Later in the scene, Casca describes to Cassius and Brutus how Antony offered the crown to Caesar three times, which Caesar refused three times, but to Casca it was all just a show and a sham.
CASCA. ... It was mere foolery; I did not mark it. I saw Mark Antony offer him a crown ... as I told you, he put it by once. But for all that, to my thinking, he would fain have had it. Then he offered it to him again; then he put it by again. But, to my thinking, he was very loath to lay his fingers off it. And then he offered it the third time; he put it the third time by. (1.2.240-247)
Casca even remarks that he almost laughed at the spectacle of it all. Casca believes that Caesar is simply play-acting and that Caesar is trying to convince the people of Rome that he wasn't ambitious for the crown, when in fact he wanted to be king above all. Cassius starts to view Casca as a like-minded Roman who might be convinced to join the conspirators against Caesar.
That evening there's a tremendous storm, and Cassius meets Casca in the street. Cassius tells Casca that the storm is an ill omen that foretells great upheaval in Rome if Caesar's lust for power goes unchecked. Casca believes that what Cassius says might well be true, and he's wary of Caesar's ambition.
CASCA. Indeed they say the senators tomorrow
Mean to establish Caesar as a king ... (1.3.91-92)
Cassius says that he would rather kill himself than let that happen.
CASSIUS. I know where I will wear this dagger then:
Cassius from bondage will deliver Cassius.
Therein, ye gods, you make the weak most strong;
Therein, ye gods, you tyrants do defeat.
... If I know this, know all the world besides,
That part of tyranny that I do bear
I can shake off at pleasure. (1.3.95-98, 104-106)
Casca shows that he's willing to bear arms against Caesar's tyranny, as he believes any man should.
CASCA. So can I.
So every bondman in his own hand bears
The power to cancel his captivity. (1.3.107-109)
Casca agrees to do whatever Cassius believes must be done, including assassinate Caesar.
CASCA. ... Hold, my hand.
Be factious for redress of all these griefs,
And I will set this foot of mine as far
As who goes farthest. (1.3.124-127)
Early the next day, Casca meets with Cassius and the other conspirators. Brutus joins the conspiracy, and the conspirators resolve to assassinate Caesar.
The assassins arrive at the Capitol with Caesar. At first there's some confusion involving Artemidorus, a fortuneteller who wants to give Caesar a letter warning him about the assassination. Cassius is concerned that the conspiracy will be exposed, and urges Casca to move towards Caesar.
Be sudden, for we fear prevention. (3.1.21-22)
Brutus urges restraint, however, and the situation resolves itself. Trebonius pulls Mark Antony aside so Antony won't interfere with the assassination, and the assassination goes forward.
BRUTUS. He is address'd; press near and second him.
CINNA. Casca, you are the first that rears your hand. (3.1.33-34)
It's been arranged that Casca will be the first to attack Caesar. The conspirators slowly surround Caesar, talking to him and distracting him, until Casca cries out as he strikes the first blow.
CASCA. Speak, hands, for me! (3.1.85)
Mark Antony remarks later in the play how "damned Casca, like a cur, behind / Struck Caesar on the neck" ( 5.1.46-47).
Casca survives the play, but the real Casca committed suicide just two years later after the Battle at Philippi, when an army led by Brutus and Cassius was defeated by an army commanded by Mark Antony and Octavian.