Cassius proves himself, by the end of Act One, to be a man of action.
First, he has a very long conversation with Brutus, in which he not only plants the seed in Brutus of revolt (and assassination), but he explains his own personal grievances with Caesar. He exposes the great Caesar as a coward in describing how he, Cassius, saved Caesar from drowning. After which, Caesar could only whimper and moan like "a sick girl." He poo-poos Caesar's greatness with mocking:
Why, man, he doth bestride the narrow world
Like a Colossus, and we petty men
Walk under his huge legs and peep about
To find ourselves dishonourable graves.
Once Brutus is baited, Cassius draws Caesar's attention by his suspicious "lean and hungry look," confirms Casca's participation in the plot against Caesar, and confesses to the audience (Scene ii, lines 312 to end of scene) that he doesn't really respect Brutus all that much, just sees him as a good ally to achieve his ends since he is so respected by Casear.
Pshew. That Cassius is a doer, and really sets the action in motion in Act One.
Cassius loves money. He is a miser. His quarrel with Brutus in Act 4, Scene 3 erupts because he failed to send Brutus the money he needed to pay his troops. Brutus says:
Let me tell you, Cassius, you yourself
Are much condemn'd to have an itching palm,
When the two men have vented all the angry feelings, Brutus calls for a bowl of wine to share with Cassius. Cassius shows his greedy character when he 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 a universal trait of misers. They are freeloaders. Cassius cannot drink too much of Brutus' love--or drink too much of his wine, either! Brutus undoubtedly provides better wine than Cassius ever buys for his own consumption.
Caesar tells Antony in Act 1, Scene 2:
Let me have men about me that are fat,
Sleek-headed men, and such as sleep o' nights:
Yond Cassius has a lean and hungry look;
He thinks too much; such men are dangerous.
Another revealing trait of misers is that they are typically lean and hungry looking. This is because they hate to spend money even on buying food for themselves. No doubt everybody in Cassius' household, especially the slaves, looks equally lean and hungry. When Cassius invites Casca to supper, Casca obviously doesn't want to come. He has been there before and knows what kind of supper he can expect: some bread and cheese and bitter-tasting wine. Then when Cassius invites him to dinner, Casca sees he can't get out of it and replies, very rudely:
Ay, if I be alive, and your mind hold, and your dinner
worth the eating.
These men have known each other since their school days. When Casca says, "...and your dinner worth the eating" he really means it. It is noteworthy that Cassius, who is anxious to talk to Casca alone, first invites him to supper. This is a light evening meal which wouldn't cost Cassius much to serve. When Casca turns him down, Cassius, being a miser, thinks he must spend a little more and provide a whole dinner. The fact is that Casca doesn't really want to come at all. People like Brutus, but they dislike Cassius for obvious reasons, including the fact that he has a terrible temper. This is why Cassius needs Brutus to act as leader of the conspiracy.
Cassius was a schemer and an instigator of Caesar’s assassination. When he talks to Brutus he understands his personal conflicts but pretends to ignore them. He instead urges Brutus to make a decision, one which is in alignment with his own, to kill Caesar. Cassius is full of praise for Brutus and tells him that even the most respected men in Rome see him as a worthy leader, except Caesar, who is his friend. The praise he accords to Brutus, although true, was meant to further drive a wedge between the two friends. Cassius also displays his jealous nature when he tells Brutus that Caesar is a coward and does not deserve to become their leader. He also talks of Caesar’s illness and that he is not strong enough to lead and with all his sentiments he is out to discredit Caesar.
At the end of act 1, Cassius was able to recruit and mobilize the conspirators who eventually participated in the assassination.