In Act 1, Cassius uses flattery to gauge Brutus's willingness to join the conspiracy. Cassius expresses disgust that Caesar is so revered by the Romans, and he makes the point to Brutus that the names "Brutus" and "Caesar" are equal (and, therefore, that Caesar is no better than Brutus and doesn't deserve the power he's enjoying). Further, Cassius reminds Brutus that his own ancestors were responsible for founding the Roman republic:
O, you and I have heard our fathers say
There was a Brutus once that would have brooked
Th' eternal devil to keep his state in Rome
As easily as a king.
Later in Act 1, Cassius further manipulates Brutus by writing letters (he uses different handwriting to make it appears as if the letters have come from various citizens) that express concern about Caesar's power. Cassius leaves the letters for Brutus to find, because he knows that Brutus's love for Rome and its citizens will make him act in opposition to Caesar.
I would argue that Cassius is not able to manipulate Brutus. He attempts to manipulate him through flattery and even some bullying, but it seems to me that Brutus is the one that gets his way. Two important places where Brutus overrides Cassius are in Act II, scene i, when the conspirators meet and Act III, scene i when he insists that Antony be allowed to speak at Caesar's funeral.
In Act II, scene i, first Brutus repudiates Cassius' suggestion that they "swear [their] resolution" to murder Caesar. Brutus has quite a long response to this in which he asserts that they are Romans and, as such, have no need to swear. It is their honor that will create their bond. And then, more crucially, when Decius Brutus suggests that they murder Antony too, Brutus puts the nix on that as well:
Let Antony and Caesar fall together.
Our course will seem too bloody, Caius Cassius,
To cut the head off and then hack the limbs...
For Antony is but a limb of Caesar.
This, along with Brutus overriding (once again) of Cassius after the murder of Caesar and permitting Antony to speak at the funeral, could be argued as the two fatal mistakes to the success of the conspiracy. In Act III, scene i:
You know not what you do. Do not consent
That Antony speak in his funeral...
It shall advantage more than do us wrong.
I know not what may fall. I like it not.
Well, like it or not, it is not Cassius that has the upper hand, much to his own frustration. So, I would argue that Cassius, try though he might, is not able to manipulate Brutus at all.
Cassius tried to flatter Brutus in order to make Brutus to join him in the conspiracy as he dislikes the fact that Caesar had become a god like figure in the Roman's eye. Brutus said he was at war with himself which made it easier for Cassius to minipulate him and play mind games with Brutus.
1) First , Cassius reminds Brutus that his own ancesstors were responsible for founding the roman republic.
2) Second of all, he tells Brutus that his name is as fair as Caesar's.
3) He tells Brutus that Caesar is just as human as they can bare the cold and so can Caesar.
4) Cassius tells Brutus how he himself saved Brutus's life when Caesar had dared him to swim across the tiger with him and started sinking.
5) Cassius also tells Brutus about the downfall of Caesar's health to prove he is as human as both of them.
6) Cassius asks Brutus to think why Caesar's name shoulb be sounded more than his whereas both their names are a heavy and why Caesar has grown so great being no different from them.
These things hit Brutus and he tells Cassius he will consider everything he has told him and willingly want to hear more.
are as heavy*