Cassius successfully persuades Brutus into joining the conspirators by appealing to his honorable character and portraying Julius Caesar as an ambitious, undeserving politician, who will eventually disband the Senate and rule Rome as a ruthless tyrant. In act 1, scene 2, Cassius recognizes that Brutus is concerned about Caesar's growing popularity and calls attention to his feelings. Cassius then mentions that many Romans wish that Brutus would intervene in government issues and recognize the threat of impending tyranny. Cassius then shifts Brutus's attention to Caesar's flaws and tells a story about the time he saved Caesar from drowning in the Tiber River. He proclaims that Caesar is a flawed mortal like everyone else and recalls a time when Caesar suffered an epileptic seizure. After Cassius focuses on Caesar's obvious flaws, he proceeds to describe Caesar's confident, arrogant personality by telling Brutus,

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 dishonorable graves. Men at some time are masters of their fates. The fault, dear Brutus, is not in our stars But in ourselves, that we are underlings (Shakespeare, 1.2.136–143).

Cassius's words are moving and depict Caesar as a domineering, arrogant man, who believes he is inherently superior. He also depicts Rome as full of passive, cowardly men, who are afraid to challenge Caesar and will not rise up against the future tyrant. Cassius then reminds Brutus of his noble, valiant ancestor by saying,

Now is it Rome indeed, and room enough, When there is in it but one only man. Oh, you and I have heard our fathers say, There was a Brutus once that would have brooked Th' eternal devil to keep his state in Rome As easily as a king (Shakespeare, 1.2.157–163).

After Cassius's moving words, Brutus responds by promising to think about his valid argument and entertains the idea of joining the conspirators. Brutus proceeds to tell Cassius,

Brutus had rather be a villager Than to repute himself a son of Rome Under these hard conditions as this time Is like to lay upon us (Shakespeare, 1.2.173–176).

In addition to portraying Caesar as an ambitious tyrant and calling attention to his obvious flaws, Cassius also forges letters from concerned citizens encouraging Brutus to intervene and preserve the Republic.

Cassius persuades Brutus against Caesar because he convinces Brutus of Caesar's tyrannical characteristics and his desire for absolute power.

When Caesar returns to Rome, having already defeated Pompey, he moves before the crowds, who want to crown him. He is offered a laurel three times, and is loath to refuse it. At this time, Cassius tells Brutus that many of the most respected Romans, "groaning under this age's yoke" (1.2.63) (the tyranny of Caesar), wish that he "had his eyes" (1.2.64); that is, they perceive better what is occurring.

Then, as Brutus hears the crowd cheering, he says that he fears Caesar has been made king by the people. Cassius replies to him that if this is what he fears, "then must I think you would not have it so" (1.2.84). Brutus replies that he loves honor more than he fears death and would like to know what Cassius has to tell him. Hearing this, Cassius takes advantage of Brutus's great love of honor, saying that "honor is the subject of my story" (1.2.94).

Cassius then launches into a soliloquy in which he describes the "feeble temper," or weak physical condition of Caesar, and he asks Brutus if such a man should become the sole leader of "the majestic world" (1.2.158) when this has not been the intention before (Rome has had triumvirates). To add emphasis, Brutus describes Caesar as a Colossus who stands over the Romans. He then reminds Brutus of his own ancestor of the same name who would not have permitted a sole king to rule:

Oh, you and I have heard our fathers say,

There was a Brutus once that would have brooked

Th' eternal devil to keep his state in Rome

As easily as a king.(1.2.160-164)

Cassius told Brutus that Caesar was too ambitious to be allowed to remain in control of Rome.

Cassius approached Brutus to make sure that he was concerned about Caesar.  He then convinced him that Caesar was dangerous, and that it was their responsibility to stop him.  He was very convincing, because he appealed to Brutus’s idealism.

CASSIUS

I know that virtue to be in you, Brutus,
As well as I do know your outward favour.
Well, honour is the subject of my story. (Act 1, Scene 2)

One of Cassius’s arguments was that he knew Caesar’s nature.  He explained to Brutus that Caesar considered himself better than everyone else in Rome, telling Brutus that Rome’s fate is in their hands.  Caesar considers himself better than everyone else.  From Cassius's perspective, Caesar's ambition will doom Rome if they let it.

CASSIUS

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.
Men at some time are masters of their fates:
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings. (Act 1, Scene 2)

This is an argument that appeals to Brutus.  He sees the people of Rome as enslaved to Caesar.  Caesar will make himself king, and by that time there will be nothing they can do to stop him.  If they are going to stop him, they need to do it soon.

Cassius does not rely on only his speeches. He makes sure that Brutus finds letters, supposedly from citizens, urging him against Caesar. These are planted, of course.  Cassius wants to make sure that he has Brutus on his side.  They need him for his name and his reputation.  Cassius appeals to this reputation, saying that Caesar’s name is no better than Brutus’s.