[By the way, this question would have been better as a Question instead of as a Topic post.] At a private meeting between Cassius and Casca, the conversation turns to the planned events of the next day when, as Casca says, "the senators tomorrow / Mean to establish Caesar as a king." In response, Cassius brings up the notion of "bondage," saying that if the crowning of Caesar comes about, he will free himself from bondage: "Cassius from bondage will deliver Cassius."
He goes further and equates bondage with Caesar by indirectly labeling Caesar as a tyrant in context of liberating himself from bondage if Caesar is crowned. Casca follows Cassius's allusions and replies with the confirmation that each bears the power to end his own captivity: "in his own hand bears / The power to cancel his captivity." Then, feigning being carried away with passion--to a Roman citizen, the thought of captivity and bandage was anathema (reprehensible)--Cassius says he can't blame Caesar for being a tyrant, or a "wolf," because the Romans were "but sheep" with no strength of mind or aim of their own:
But that he sees the Romans are but sheep:
He were no lion, were not Romans hinds.
He goes on to say that the "fire" of making of Caesar king will "illuminate / So vile a thing as Caesar!" After intentionally exposing himself this way (though pretending to be carried away), Cassius says, "But, O grief," saying that perhaps Casca is loyal to Caesar and wants him to be king. Here, Casca replies that he too thinks Caesar's ambition should be halted ("Be factious for redress of all these griefs,") and makes a pact ("Hold, my hand") to go to the farthest limits with Cassius against Caesar, which would imply going so far as to take Caesar's life: "And I will set this foot of mine as far / As who goes farthest." This is how Cassius draws Casca into the conspiracy.
Cassius is obviously the ringleader of the plot to assassinate Julius Caesar. When that event takes place, Caesar is surrounded and stabbed by many individuals. Presumably they have all been recruited by Cassius. However, Shakespeare could not devote the time to showing Cassius approaching all these conspirators one by one and using his skills at persuasion. Besides that, Cassius has to devote much of his thought and effort to persuading Brutus to turn against Caesar and become the titular leader of the conspiracy. For these reasons Casca plays a prominent role in the play. Casca represents all the other men Cassius recruits. Otherwise, Casca is not an impressive character but strikes the viewer as being rather common, superstitious and intellectually limited.
Casca is rude to Cassius when Cassius invites him to dinner. He says:
Ay, if I be alive, and your mind hold, and your dinner worth the eating.
Casca doesn't like Cassius. He obviously doesn't want to have dinner with him. One of the reasons he dislikes Cassius is that he knows him to be a selfish miser. This trait will be reflected in everything Cassius subsequently does, including withholding the gold Brutus requests to pay his troops. Casca does not expect the dinner to be very good. Cassius will be serving third-rate wine with a bitter aftertaste. The portions will be small, and the meat will be tough. Casca has known Cassius since boyhood and knows what to expect of his hospitaity. He also knows that Cassius wouldn't be giving him anything if he didn't expect to get more in return.
But what is important here is that Casca is being used as a representative of all the men Cassius is trying to draw into his plot against Caesar. The viewer should understand that Cassius has been busily inviting other men to his suppers and dinners, where he can sound them out in confidence about their feelings. They are all limited individuals, like Casca himself, only suited to be followers; and they all have the same unfavorable opinion of Cassius. That is why Cassius knows he has to have Brutus on his side.
Brutus is known to be a noble and honorable man. Many people would follow him who would not follow Cassius. If and when the assassination attempt succeeds, a new power structure will have to be established. Nobody wants to see Cassius taking Caesar's place, because he might turn out to be a worse tyrant than the man they got rid of; but they would be happy to see Brutus doing so, because Brutus thinks about the common good and not about his private advantage. When Cassius succeeds in winning Brutus over to his cause, he has no trouble recruiting Casca and all the others.
In what is known as the "seduction scene" of Act I, Scene 2, as Cassius speaks with Brutus in his effort to enlist him in the conspiracy to assassinate Julius Caesar, he elicits the testimony of Casca as further evidence of Caesar's tyranny and desire for absolute power. At first Casca wonders why Cassius asks him to tell Brutus about what has happened when Marc Antony offered Caesar the crown, saying, "Why you were with him, were you not?" But, cleverly Cassius encourages Casca to speak asking him pointed questions such as "Was the crown offered to [Caesar]three times?" When Cassius asks Casca to relate the details, Casca at first says that he did not really pay attention; however, as he narrates the details, Casca begins to think about the circumstances and he remarks that Caesar "would fain have it." Thus, Cassius has planted the seed of thought that Caesar is, indeed, power hungry.
Further, Cassius has Casca relate how Caesar fainted; this is another ploy to get Casca to think that Caesar is physically inferior to what a ruler should be. For, by this time in his narration of what has occurred, Casca has grown angry, saying "marry" which is an expletive. He becomes disparaging of Caesar, too, as he remarks that Caesar wanted people to think that it was his infirmity which made him fall because they were so in love with him that they forgave him. Angrily, Casca tells Cassius that Marius and Flavius have been killed and there was even more "foolery yet" if he could remember.
Of course, these words make an impression upon Brutus, who has been listening, but Casca himself has unconsciously been swayed against Julius Caesar.