At the beginning of the play, Cassius is already scheming against Caesar and trying to talk Brutus into joining a conspiracy against him. Brutus reveals that he too is worried that Caesar is becoming too powerful and must be stopped. But, in his private conversation with Cassius, he stops short of actually declaring himself willing to take action. Cassius devises a plan to persuade Brutus to take the crucial step of joining the conspiracy.He decides to forge some letters, to make it seem that they have been written by Roman citizens urging action against Caesar, and throw them in at Brutus's window. Cassius is sure that this will sway Brutus as Brutus has idealistic notions about acting for the greater good of the people. Cassius is correct; this does finally persuade Brutus. We see how manipulative Cassius is here, not hesitating to resort to tricks to get Brutus on his side, as he thinks that Brutus, being generally well-liked and respected in Rome, will help make the conspiracy more respectable.
It is clear from the outset that Cassius resents Caesar and feels that he is not worthy of the status he is being given. Cassius himself is hungry for power and believes that he has greater abilities than Caesar. The fact that the general populace seems to treat Caesar as if he is some kind of god greatly troubles Cassius. He is intent on preventing the general from achieving even greater power and has, with a number of other conspirators who share his sentiment, decided to get rid of him.
The first part of their plot is to get the ordinary citizens to turn against Caesar, as is displayed by the actions of Marullus and Flavius at the beginning of the play. They go around encouraging others to deface statues put up in honor of the general and remove whatever tokens of praise these effigies have been decorated with. Furthermore, they criticize others' praise of Caesar and remind them of Pompey's greatness. It is obvious that they wish to turn as many of the citizens as possible against the general and start a rebellion.
Cassius approaches Brutus since he knows that Brutus shares a close bond with Caesar and also that Brutus has great esteem in the eyes of the general public and other senators. He is respected for being honorable and brave. If he should persuade Brutus to join his conspiracy, it would give validity to the plot. Everyone would understand that Brutus would never act out of greed or selfish desire and would, therefore, support him.
Cassius's conversations with Brutus pertinently display his resentment for the general. He continuously reminds Brutus of how weak Caesar actually is and recalls incidents when Caesar's frailties were exposed. He then juxtaposes these with his and Brutus's strengths and especially plays on the latter's sentiment by waxing lyrical about his noble qualities. In this, he attempts to sweet-talk Brutus into joining them in the plot against Caesar.
Brutus promises to consider what Cassius has told him for he also believes that there is some risk in Caesar becoming emperor. At the end of their conversation, he tells Cassius that they will speak again. Cassius is confident that he has baited Brutus enough and urges others to plant letters, supposedly written by ordinary citizens, in Brutus's chambers, which would urge him to take action against Caesar. His machinations are quite effective because, in the end, Brutus joins the conspiracy.
Knowing how important the state of Rome is to Brutus, and his willingness to appease its people, Cassius creates a fake petition and throws it through Brutus's window. This petition looks as if it is from citizens who are unhappy and demanding that Caesar be removed.