The most significant characteristic of Cassius is his ability to perceive the true motives of men. Caesar says of him, "He reads much; / He is a great observer and he looks / Quite through the deeds of men." The great irony surrounding Cassius throughout the play is that he nullifies his greatest asset when he allows Brutus to take effective control of the republican faction.
Cassius believes that the nobility of Rome are responsible for the government of Rome. They have allowed a man to gain excessive power; therefore, they have the responsibility to stop him, and with a man of Caesar's well-known ambition, that can only mean assassination.
Cassius intensely dislikes Caesar personally, but he also deeply resents being subservient to a tyrant, and there are indications that he would fight for his personal freedom under any tyrant. He does not resent following the almost dictatorial pronouncements of his equal, Brutus, although he does disagree heatedly with most of Brutus' tactical decisions. To accomplish his goal of removing Caesar from power, he resorts to using his keen insight into human nature to deceive Brutus by means of a long and passionate argument, coupled with bogus notes. In the conversation, he appeals to Brutus' sense of honor, nobility, and pride more than he presents concrete examples of Caesar's tyrannical actions. Later, he is more outrightly devious in the use of forged notes, the last of which prompts Brutus to leave off contemplation and to join the conspiracy. Cassius later uses similar means to bring Casca into the plot.
Throughout the action, Cassius remains relatively unconcerned with the unscrupulous means he is willing to use to further the republican cause, and at Sardis, he and Brutus come almost to breaking up their alliance because Brutus objects to his ways of collecting revenue to support the armies. Cassius sees Brutus as the catalyst that will unite the leading nobles in a conspiracy, and he makes the recruitment of Brutus his first priority. Ironically, his success leads directly to a continuous decline of his own influence within the republican camp.
Clearly, Cassius has his negative aspects. He envies Caesar; he becomes an assassin; and he will consent to bribery, sell commissions, and impose ruinous taxation to raise money. But he also has a certain nobility of mind that is generally recognized. When Caesar tells Antony that Cassius is dangerous, Antony answers, "Fear him not, Caesar; he's not dangerous. / He's a noble Roman and well given." He was no doubt expressing sentiments popular at the time. Cassius is also highly emotional. He displays extreme hatred in his verbal attack on Caesar during Lupercal; he almost loses control because of fear when Popilius reveals that the conspirators' plans have been leaked; he gives vent to anger in his argument with Brutus in the tent at Sardis; he expresses an understanding tolerance of the poet who pleads for him and Brutus to stop their quarrel; and he threatens suicide repeatedly and finally chooses self-inflicted death to humiliating capture by Antony and Octavius. When he becomes a genuine friend of Brutus following the reconciliation in the tent, he remains faithful and refuses to blame Brutus for the dilemma that he encounters at Philippi, even though he has reason to do so.
Of all the leading characters in Julius Caesar, Cassius develops most as the action progresses. At the end of Act I, Scene 2, he is a passionate and devious manipulator striving to use Brutus to gain his ends. By the end of Act IV, Scene 3, he is a calm friend of Brutus who will remain faithful to their friendship until death.