Cassius convinces Brutus to join the conspiracy by appealing to his honorable nature and arguing that Rome should not be ruled by one man. Cassius recognizes that Brutus is an honorable politician, who is primarily concerned with the well-being of the Roman population. Unlike the other selfish senators, Cassius understands that Brutus will be motivated to preserve the Republic and protect the citizens from tyranny.

In act 1, scene 2, Cassius begins to convince Brutus by portraying himself as an honest, trustworthy friend, who is looking out for his best interests. After Brutus admits that he doesn't want Caesar to be king, Cassius comments that he would rather not live than worship a man as ordinary as himself. Cassius then proceeds to highlights Caesar's flaws by telling a story of how he saved him from drowning in the Tiber and recalling Caesar's epileptic fits. Cassius goes on to question why Caesar should be viewed as a Colossus while "petty men" walk under his legs. He then tells Brutus,

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 (1.2.140–143).

Cassius proceeds to question why Rome should be ruled by one man while the rest of the population serves his interests. Once again, Cassius appeals to Brutus's nobility by saying, "Rome, thou hast lost the breed of noble bloods!" (1.2.152). He then brings up Brutus's famous ancestor, who defeated Rome's last tyrant, Lucius Tarquinius Superbus. Since Cassius is suggesting that Caesar will become a tyrant, he hopes that Brutus will live up to his ancestor's reputation and take action. In addition to Cassius's persuasive argument against Caesar's tyranny, he also has fabricated letters written from concerned citizens sent to Brutus's home. Cassius recognizes that Brutus will defend the Republic and protect the citizens at all costs, which is why he sends the letters.

Cassius persuades his friend Brutus to join in on the conspiracy to murder Caesar by playing upon his friend's committed love for Rome as a democratic place, which Cassius believes Caesar is destroying. He points out that Rome has changed considerably under the rule of Caesar, asking whether there has ever before been a time when the idea of Rome "encompass'd but one man." Cassius is suggesting that Caesar has become far too powerful and that the figurehead he has become is something contrary to the idea of Rome and what it is.

Cassius points out to Brutus that even though he may love Caesar, it is actually a matter of family honor that he should recognize the importance of placing Rome before his friendship with a mere man—and an increasingly corrupt one, at that. Cassius notes that an earlier member of Brutus's family would have "brooked the eternal devil" in order to keep Rome safe from tyranny. That being the case, now that Caesar has essentially "become a god," it is important that Cassius should step in and help to challenge this. After all, Brutus and Cassius have known Caesar as a person for a long time; they know that he is, for example, not a strong swimmer and once required their help to prevent him drowning. For him to set himself up as a god, then, is ludicrous. He must be stopped.

In act one, scene two, Cassius tells Brutus to let him be his "glass," meaning his mirror to better see himself. He then proceeds to interpret, for Brutus, all of Brutus's reactions to what they can hear of Caesar's interaction with the citizens. When Brutus says that he fears that "the people / Choose Caesar for their king," Cassius replies that this must mean that Brutus recognizes that Caesar should not be "their king." Brutus, like Cassius, is concerned that Caesar is becoming too powerful.

Cassius then tells Brutus that they are as worthy as Caesar, that they "both have fed as well" and were "born free as Caesar." The implication that Cassius is trying to push here is that they are as worthy as Caesar and, therefore, Caesar should not be more powerful than they are. Cassius continues to mock Caesar's masculinity or supposed lack thereof. He relates a story in which Caesar was drowning and asked for Cassius's help and then says, incredulously, that this same Caesar "is now become a God." Again, Cassius is trying to persuade Brutus that Caesar has become too powerful and is underserving of this power. To emphasize the point, he calls Caesar a "coward" and compares him to "a sick girl."

As persuasive as Cassius's arguments are up to this point, he really hits a nerve, so to speak, when he questions what Caesar is doing to Rome. Brutus truly loves Rome and eventually agrees to join the conspiracy because he genuinely believes that Caesar must be removed for the benefit of his beloved Rome. Cassius points out that Rome has never been ruled by just one man before:

When could they say till now, that talk'd of Rome,
That her wide walls encompass'd but one man?

Indeed, before Caesar, Rome was ruled by three men. Cassius suggests that now Rome is becoming something like a dictatorship. Finally, at the end of their conversation and just before Caesar re-enters, Cassius invokes the name of Brutus's relation, who, long ago, once fought to save Rome:

There was a Brutus once that would have brook'd
The eternal devil to keep his state in Rome

In summary, Cassius persuades Brutus to join the conspiracy against Caesar firstly by highlighting how much power Casear has accrued and then by questioning his right to this power given that he is no better than them. Cassius then says that Caesar is in fact less than them—he is cowardly and weak. He then suggests that Caesar is endangering Rome, which Brutus loves and is loyal to, and finally he implies that it is also, for Brutus, a question of family honor to protect Rome.