Cassius is already scheming against Caesar and trying to talk Brutus into joining a conspiracy against him. Brutus 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.
