There are four main characters in Julius Caesar: Caesar, Antony, Brutus, and Cassius. Three of these are very similar in being strong, brave, self-sufficient, independent, and "noble." By the word "noble" must have been meant statesmanlike, concerned for others, generous. The only exception is Cassius, who is portrayed by Shakespeare as greedy and selfish. Shakespeare has Cassius himself admit that he is not noble but ignoble in his soliloquy in Act 1, Scene 2.
Well, Brutus, thou art noble; yet, I see,
Thy honourable metal may be wrought
From that it is disposed: therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced?
Caesar doth bear me hard; but he loves Brutus:
If I were Brutus now and he were Cassius,
He should not humour me.
Cassius seems to be saying that Brutus is noble but can be induced to commit an ignoble action, which is to participate in the assassination of his friend Caesar.
Although Cassius is the least admirable of the four main characters, it is he who conceives and organizes the plot to kill Caesar. Without Cassius there would have been no assassination, and history would have been different. Caesar, Antony, and Brutus are not afraid to act openly, but Cassius has to operate in secret and often under the cover of darkness. Shakespeare must have intended that his audience would end up sympathiziing with Caesar and his avengers, Antony and Octavius, since he took pains throughout the play to make Cassius--though just as courageous as the other three men--cunning, greedy, miserly, self-centered, and treacherous.
In Act 1, Scene 2, Cassius is trying desperately to persuade Brutus to join the conspiracy as its leader. Cassius knows that people do not respect him enough to follow his lead. They do not respect him because they know him too well. In Act 1, Scene 2, Shakespeare purposely inserts a few lines of dialogue between Cassius and Casca to illustrate the disdain that the nobles in general feel for Cassius.
Will you sup with me to-night, Casca?
No, I am promised forth.
Will you dine with me to-morrow?
Ay, if I be alive and your mind hold and your dinner
worth the eating.
Good: I will expect you.
Cassius wants to get Casca alone in order to persuade him to join his conspiracy. Casca might be said to represent all the aristocrats Cassius is trying to enlist. It would be too complicated to show Cassius talking to many different acquaintances, so Shakespeare encapsulates the whole process into this one tiny "bit."
Cassius show his miserly character by suggesting supper, which is a light meal probably consisting only of wine, cheese, and a little fruit. When Casca tries to back out, Cassius offers dinner. Casca is a blunt fellow but not always so deliberately rude. He accepts angrily. He doesn't like being pressured, and he knows what sort of wine and food to expect, since he has known Cassius since they were schoolboys. He obviously dislikes Cassius--and this suggests why Cassius has to draw Brutus into his plot.
With Brutus as the leader of the conspiracy, many others will eagerly join in. Brutus is not only highly regarded by everyone, but, since he is a friend of Caesar, his willingness to participate in an assassination plot will make it seem like the just and necessary thing to do.
Brutus is an introverted, unworldly, bookish philosopher. Unlike others such as Casca, he trusts Cassius and believes him to be as honorably motivated as himself.