At this point, Cassius appears very devious, playing the role of the scheming villain to perfection as he plots to sway Brutus to join his side against Caesar once and for all. He knows Brutus is wavering but still will not be easy to persuade.
Cassius has already stated his own arguments as to why Caesar should be eliminated, but he knows that Brutus is more likely to be persuaded by an appeal from the people of Rome. Brutus is a political idealist, always thinking about the greater good of Rome as a republic, where no one man should be allowed too much power, as Caesar appears to have.
Therefore Cassius decides to forge letters as though they came from the people of Rome and throw them in at Brutus's window. These letters will not only speak of the possible dangers that Caesar's ambition poses to the state, but will also flatter Brutus. Cassius is pretty sure that Brutus won't be able to resist this and he is proved right.
Brutus, unlike Cassius, has a noble reputation, but he is also somewhat gullible, and so he is quite easily tricked into believing that the ordinary people of Rome are directly appealing to him through these letters to save the state from Caesar. It is after reading these letters that he finally decides to join the conspiracy. The wily Cassius gets his way.