How does Cassius's diction allow him to persuade Brutus to his cause?
In many ways, Julius Caesar is a play that is concerned with speech and how it is used to manipulate, coerce and flatter people. Speech is a tool used by characters to get other characters to do what they want, and there are many victims of speech used in this way, for example Caesar himself and also Casca. One of the prime examples though, is this section of the play in Act I scene 2 where Cassius "sounds out" Brutus and then persuades him to join the plot against Caesar.
It is well worth examining Cassius' strategy in how he does this. He starts of by commenting that he has noticed a change in Brutus' regard towards him, recognising that there is something going on within Brutus. Obviously, one of the aspects of Cassius is that he is a very good judge of character, as Caesar goes on to recognise. He then goes on to flatter Brutus, assuring him of how high he stands in favour with the people and the Senate:
I have heard
Where many of the best respect in Rome
(Except immortal Caesar), speaking of Brutus,
And groaning underneath this age's yoke,
Have wished that noble Brutus had his eyes.
Notice how subtly Cassius introduces Caesar's name here, in parenthesis and with the sarcastic title "immortal" to suggest an opposition. He then assures Brutus of his honesty and gives testament to his upstanding character to encourage Brutus to believe his words.
A key point in the discussion comes when they hear a flourish and a shout (as said in the stage directions). Brutus, already goaded into thought by Cassius, says:
I do fear the people
Choose Caesar for their king.
This word "fear" allows Cassius to talk about Caesar's humanity and how now he is passing himself off as divine:
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 ourselves dishonourable graves.
Note here again the irony in the description of "petty men" - the juxtaposition of Cassius' description of Caesar as just another mortal man and then this description of the God-like Caesar obviously undercuts Caesar's divinity and draws attention again to how one man has seized power and put others (like Brutus) under him. Note how this speech progresses by introducing an element of jealousy and unfairness into what has happened:
Brutus and Caesar: what should be in that "Caesar"?
Why should that name be sounded more than yours?
Having introduced (rather serpent-like) this temptation with logical reasoning, Cassius goes on to refer to Brutus' ancestor (also called Brutus) who was involved in deposing the last tyrant of Rome, obviously trying to goad Brutus into action by being true to his ancestors and heritage.
Notice how when he has acheived his objective, Cassius is self-deprecating about his talents:
I am glad
That my weak words have struck but thus much show
Of fire from Brutus.
Here we have an example of a master in persuasion - Cassius' words are anything but "weak", and he establishes himself in this scene as a key manipulator and user of rhetoric.