Cinna and Cassius hope to win Brutus to their cause. They need Brutus to join the conspiracy because he is very well respected in Rome, and the Roman people are more likely to accept Caesar's assassination if Brutus is a part of it. Cinna encourages Cassius to enlist Brutus's participation:
O Cassius, if you could
But win the noble Brutus to our party--
O, [Brutus] sits high in all the people's hearts;
And that which would appear offense in us,
His countenance, like richest alchemy,
Will change to virtue and to worthiness.
Cassius knows they must win Brutus over. Besides talking to him about the cause of the conspirators, Cassius forges letters to Brutus, letters that supposedly were written by the Roman people begging Brutus to free them from Caesar. Cinna delivers the letters, and Brutus finally joins the conspiracy and takes part in Caesar's murder.