Cassius is the main conspirator who convinces Brutus to join the conspiracy, even though Brutus must come to the realization that while he loves Caesar, he loves his country (Rome) more before he agrees to become part of the plot to end the life of his leader.
Cassius uses an array of rhetoric, including a tale in which Caesar and he are swimming across the river, and while he makes it fine, he has to go back and rescue Caesar who isn't strong enough to fight against the current and bear the weight of his own armor. Cassius claims that Caesar is too weak, and his envy/jealousy shines through in his examples. However, Brutus eventually weighs the pros and cons, and decides that Caesar truly is a danger to the system of government and way of life that his country enjoys currently.
Cassius convinces Brutus to join the conspiracy. He does this so that Brutus joins the conspiracy to kill Caesar. He knows that Brutus is a respectable man and that the people of Rome like him and if he were to join them, the people would not react with violence and want to kill them. He claims that Caesar is not a good leader and is far too weak to rule Rome.