Act I, Scene II: "Caesar doth bear me hard, but he loves Brutus. If I were Brutus now and he were Cassius, he should not humor me. I will this night, in several hands, in at his windows throw, As if they came from several citizens, writings all tending to the great opinion that Rome hold of his name, wherein obscurely Caesar's ambiion shall be glanced at" (lines 308-315).
Since Cassius knows his own words aren't enough to convince Brutus to join the conspirators, he has decided to write fake letters and forge other citizens' names to them to urge Brutus further.
Act I, Scene II: "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 outselves dishonorable graves. Men at some point are masters of their fates. The fault, dear Brutus, is not in our stars but in ourselves, that we are underlings."
Cassius is making Brutus seem like they are Caesar's slaves when the reality is they are two well respected and wealthy Sentors in Rome.
Towards the end of Act I, Sc. ii, when left alone Cassius gives a very revealing soliloquy in which he muses on how even 'honourable' people like Brutus can be persuaded to act in a less noble fashion:
For who so firm that cannot be seduced?
This quote shows how he himself is trying to manipulate Brutus into joining the conspiracy against Caesar and agreeing to his (Caesar's) murder. He has, in effect, been trying to 'seduce', or persuade Brutus to join this dubious cause of assassinating the most prominent figure in Rome. Throughout this scene he has been putting forward all kinds of arguments as to why Caesar should be eliminated, but Brutus has not joined him as yet. Cassius now goes on to plan further on how to get Brutus on his side against Caesar. He says he'll forge letters to make it seem that the people of Rome are asking Brutus for help against Caesar. Knowing how Brutus idealistically believes in the greater good of Rome, Cassius is confident that this final, faked appeal will finally sway Brutus over to the anti-Caesar conspiracy.