The friendship between Brutus and Cassius is not particularly cordial when we first meet them. They associate with each other because they are brothers-in-law. Cassius is married to Brutus' sister. Since these two characters are going to be so important to the play and so closely associated together, Shakespeare felt it necessary to distinguish them with obviously contrasting personalities. Cassius shows that he is cunning, calculating, manipulative, envious, and miserly. He reveals his true self after the two men separate at the end of Act 1, Scene 2. He says to himself:
Well, Brutus, thou art noble; yet, I see
Thy honorable metal may be wrought
From that it is disposed; therefore it is meet
That noble minds keep ever with their likes;
For who so firm that cannot be seduced?
Caesar doth bear me hard, but he loves Brutus.
If I were Brutus now and he were Cassius,
He should not humor me. I will this night,
In several hands, in at his windows throw,
As if they came from several citizens,
Writings, all tending to the great opinion
That Rome holds of his name, wherein obscurely
Caesar's ambition shall be glanced at.
And after this let Caesar seat him sure;
For we will shake him, or worse days endure.
Brutus, by contrast, is everything Cassius is not. Brutus is a philosopher and a patriotic Roman. He is a bookish man, and like most such men he is not worldly wise or good at judging others. He misjudges Cassius, and later he spoils the whole conspiracy by trusting Marc Antony. Brutus and Cassius should not be friends. They become sincerely friendly at the end of the play because they have been through so much together and are facing a battle which could result in total disaster for both of them. Cassius has been friendly towards Brutus because he wants to use him. Brutus has reciprocated Cassius' friendliness because he believes this deceptive man is actually sincere. Sincerity is not a part of Cassius' makeup, whereas sincerity means everything to Brutus. He is incapable of deceit, while Cassius is incapable of honesty and can't even understand it. The bitter quarrel these two men have in Act 4, Scene 2 was inevitable, since they are such different types. Brutus realizes that he totally misunderstood Cassius' character, while Cassius realizes that Brutus is not as easy to manipulate as he had expected. They become friends on the battlefield at Philippi because they have no other friends in the entire world. It was an unfortunate partnership from the beginning.