Casca seems bitter and sarcastic, and clearly detests Caesar. When he recounts the story of Antony offering Caesar the crown at the Lupercal, he makes it clear that he believes that Caesar really wanted to take it. "To my thinking," he says, "he was loath to take his fingers off it." He also says he would have liked to cut Caesar's throat himself, and is very contemptuous of his appeals to the mob. In the third scene, we see that he is very superstitious, which basically just makes him a man of his time. He is terrified by the evil omens he detects, and fears for what is about to happen:
I have seen tempests, when the scolding winds
Have rived the knotty oaks, and I have seen
The ambitious ocean swell and rage and foam,
To be exalted with the threatening clouds,
But never till tonight, never till now,
Did I go through a tempest dropping fire.
Either there is a civil strife in heaven,
Or else the world too saucy with the gods
Incenses them to send destruction.
Cassius dismisses his fears, and by the end of the scene has persuaded Casca to join him in resisting tyranny. Casca says that in pursuit of liberty, "I will set this foot of mine as far
As who goes farthest."
When Casca says, "I will set this foot of mine as far / As who goes farthest," he shows that he is courageous but also that he is a follower and not a leader. He will not venture to go any farther than whoever goes the farthest. He needs someone to tell him what to do and when to do it.
Casca also seems unintelligent, which could help to explain why he is a follower and not a leader. This mental limitation shows not only in his superstitious behavior but also in his blunt way of expressing himself, which borders on outright rudeness and makes him seem more plebian than patrician. This is especially noticeable when Cassius, who is anxious to draw him into his conspiracy against Caesar, pressures him with invitations.
Will you sup with me tonight, Casca?
No, I am promised forth.
Will you dine with me tomorrow?
Ay, if I be alive, and your mind hold, and your dinner worth the eating.
This exchange tells a lot. For one thing it reveals that Casca doesn't like Cassius and doesn't want to eat with him. In contemporary cultural terms, we might say that he has perhaps sampled Cassius' hospitality in the past and perhaps knows that he can expect the social equivalent of small portions and third-rate wine.
This also helps to explain why Cassius is trying to draw Brutus into his conspiracy. Brutus is well-liked and respected by everyone, while Cassius is not popular because of his meanness, greediness, selfishness and stinginess. Cassius by himself could never lead an assassination plot against Caesar. Casca by himself would never dream of conspiring against Caesar.