This is a very interesting question, because Cassius is a character who seems to begin the play as being an incredibly strong person. However, as the play progresses he shows his weakness in the way that he concedes to Brutus, against his better judgement, and allows him to gain leadership of the conspirators.
Let us look at Cassius in Act I. He is a man who is presented as being sure of his own powers of persuasion and strength. Consider his attitude in Act I scene 2 where he so easily and expertly manipulates Brutus to join the conspirators. Cassius is presented as the leader of the conspirators and definite about what he plans to do and the impact it will have:
And after this, let Caesar seat him sure;
For we will shake him, or worse days endure.
However, what is interesting is that as the play progresses, Cassius appears to move from being an incredibly strong character who is sure of his own judgement and power, to one who constantly defers to Brutus and more of a weak character. This first begins in Act II scene 1, when Cassius asks the conspirators to make an oath, only to be overruled by Brutus and his nobility. This continues as Cassius argues that Marc Anthony must be killed, only to be overruled yet again by Brutus. In spite of the superior judgement of Cassius and his politically savvy knowledge, he remains silent and appears to be somewhat overshadowed by Brutus. Thus Cassius begins the play as a strong character, but then slowly becomes weaker as the play progresses.
Shakespeare realized he could not have too many strong characters. Caesar and Antony are both strong characters, and Cassius’s partner Brutus is obviously the stronger of those two men. In order to differentiate Cassius, Shakespeare has characterized him as a selfish and greedy miser—although no doubt he is brave, intelligent, and proud. He is a shrewd judge of men because he judges them by himself and knows himself to be selfish, cunning, and treacherous. That is why he is opposed to letting Antony speak at Caesar’s funeral. That is why he foresees that Caesar could become a tyrant. That is why Caesar is afraid of him.
There are several interesting bits of dialogue in which Cassius reveals his character.
CASSIUS Will you sup with me tonight, Casca?
CASCA No, I am promised forth.
CASSIUS Will you dine with me tomorrow?
CASCA Ay, if I be alive, and your mind hold, and your dinner worth the eating.
Casca has known Cassius since school days. He knows what kind of food and wine to expect. Cassius first invites him to supper, which is a light meal consisting of a little cheese, bread and wine and would not cost him much. Then Cassius ups the ante (judging Casca by himself) and invites him to dinner, which would of course be more elaborate and expensive. Casca obviously doesn’t like Cassius and feels pressured, as his reply indicates. He doesn’t expect Cassius’s dinner to be worth the eating. He has probably had bad previous experiences.
Another interesting exchange occurs just after the heated argument between Brutus and Cassius in Act 4, Scene 2. Brutus asks Lucius to bring him a bowl of wine and says, “In this I bury all unkindness, Cassius.” Evidently both men drink out of the same bowl. Cassius says
My heart is thirsty for that noble pledge.
Fill, Lucius, till the wine o’erswell the cup.
I cannot drink too much of Brutus’ love.
This is an unmistakable sign of a true miser. He cannot drink too much—as long as someone else is providing the wine. And he knows that Brutus provides good wine, whereas he himself serves his guests—as Casca knows-- the cheapest stuff he can buy. No doubt Cassius is just as frugal with his own consumption, like misers generally, which would help to explain what Caesar calls his “lean and hungry look.” A miser always gets more than he gives.
Another subtle sign of Cassius’s character can be detected when he and Brutus meet with Antony and Octavius on the battlefield. Cassius says
The posture of your blows are yet unknown;
But for your words, they rob the Hybla bees,
And leave them honeyless.
Such outrageous flattery just before a major battle! Cassius is obviously trying to curry favor with Antony for some selfish reason--probably hoping for favorable treatment should he be captured, or even hinting that he might be open to negotiating a separate peace with Antony and Octavius. This latter suggestion is not too far-fetched, considering that he didn’t want to fight this battle and probably doesn’t expect to win. Would he sell Brutus out? Maybe he would, after that humiliating tongue-lashing he received in Brutus’s tent, and after the way he has been consistently overruled by his partner and forced into a subordinate position when he was the one who orchestrated the entire plot against Caesar in the first place.