Cassius and Brutus are fundamentally different people who are, for political expediency, seeking the same goal: the ouster of Caesar. Brutus primarily seeks honor, nobility, and political freedom. Cassius seeks political freedom also, but unlike Brutus, he does not put honor above all else.
We see their character differences most strikingly in the tent scene in Act IV, Scene III. Brutus is infuriated that Cassius has not sent the gold he needs to pay his troops. As the argument reaches a climax, Cassius reveals that he feels a truly powerful bond of friendship with Brutus. At one point he says of himself, “Cassius is aweary of the world . . . hated by one he loves [Brutus],” and, because of their argument, “I could weep the spirit from my eyes.” Cassius shows an emotional side that Brutus does not share. He is unable to counter Brutus’ self-certainty because he is not above emotional weakness in the same way Brutus is. Brutus does not give in because he is not swayed by emotion and is certain of his own motives and judgment, although he turns out to be wrong on more than one occasion.
Cassius wanted to organize a group of Romans to assassinate Caesar. He succeeded in attracting a number of co-conspirators, but he was wise enough to realize that they needed a figurehead with a distinguished name and a sterling reputation for integrity. Cassius succeeded in recruiting Brutus, but Brutus proved to be a man Cassius could not manipulate as he had anticipated. Brutus became as headstrong and arrogant as Julius Caesar. Cassius always yielded to him because he realized that he himself did not have the charisma to be the leader of the conspirators. Cassius was not a likable person. He was not noble. He was not a natural leader. He needed Brutus, but Brutus did not need him, as Brutus clearly showed him when they had their argument in the tent in Act 4, Scene 2. Brutus says, "Away, slight man." He can lead the army without Cassius, and Cassius knows it. Cassius is virtually helpless without Brutus. After this turning-point Cassius is a meek subordinate. Brutus is a philosopher, an idealist, and quite an egotist. He is not worldly wise like Cassius. Brutus makes some serious mistakes because he fails to listen to Cassius. His biggest mistake was in not having Antony killed when they killed Caesar. His second biggest mistake was in permitting Antony to speak at Caesar's funeral. Perhaps Cassius' biggest mistake was in recruiting Brutus to lend his name to the plot against Caesar.