While I cannot write an essay on Cassius's persuasiveness for you, I can suggest you start your essay on the general theme of the power of words in Julius Caesar. There are several instances in the play where men are compelled by the power of the words of others to act with varying degrees of severity and with varying impact on the outcome of the tragedy.
Cassius in particular is constantly manipulating the early plot by pulling Brutus over to his side. He does so through flattery, manipulation (fake letters), and false comparison (Brutus's name with Caesar to show equality of character) among other tricks, as this example shows:
I would think these three headings would make solid body paragraphs for an essay, as you should be able to find plenty of textual examples from the first act to support each one.