Brutus is always portrayed as noble and magnanimous. Cassius is always portrayed as cunning and selfish. Early in the play, after feeling he has nearly persuaded Brutus to act as the leader of the conspiracy against Caesar, Cassius has a revealing soliloquy.
Well, Brutus, thou art noble; yet, I see,
Thy honourable 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 humour me. (I.2)
People tend to judge others by themselves. When Antony comes to the conspirators after the assassination of Caesar, Brutus treats him with trust and kindness, because he assumes, incorrectly, that Antony is honest and trustworthy. Cassius, on the other hand, judges Antony by himself and assumes he is comparably cunning and treacherous. Cassius wants to have him killed along with Caesar, but Brutus, with characteristic kindness and generosity, overrules him. When Antony humbly requests permission to speak in Caesar's funeral, Brutus freely grants it. Cassius is appalled.
Brutus, a word with you.
You know not what you do: do not consent
That Antony speak in his funeral:
Know you how much the people may be moved
By that which he will utter? (III.1)
Cassius thought he could use Brutus as a figurehead and that he could be the real leader of the conspiracy and the real brains behind the new government himself. Brutus was reluctant even to join the conspiracy, but once he had gotten through his soul-searching he began to change. He thought he was wiser than any other conspirator. Cassius couldn't control him. Brutus made many mistakes because he was not worldly wise like Cassius or Antony but a bookish, solitary, philosophical type of man who resembles some of Shakespeare's other unworldly characters, including Hamlet, Prospero, Duke Senior, and Richard II.
With such a difference in characters, there was bound to be a rift between the two leaders; it finally comes in the well-known "tent scene" in which Brutus asserts his supreme authority. The violent argument ends with both men badly needing a drink.
My heart is thirsty for that noble pledge.
Fill, Lucius, till the wine o'erswell the cup;
I cannot drink too much of Brutus' love. (IV.3)
Here is one of many places in which Shakespeare subtly shows Cassius's miserly character. He cannot drink too much of Brutus's love, and he cannot drink too much of his wine, either, as long as it isn't costing him anything.
The two men show their different characters throughout the play. When Antony puts himself at their mercy, Brutus nobly tells him:
O Antony, beg not your death of us.
Though now we must appear bloody and cruel,
As, by our hands and this our present act,
You see we do, yet see you but our hands
And this the bleeding business they have done:
Our hearts you see not; they are pitiful;
And pity to the general wrong of Rome--
As fire drives out fire, so pity pity--
Hath done this deed on Caesar. For your part,
To you our swords have leaden points, Mark Antony:
Our arms, in strength of malice, and our hearts
Of brothers' temper, do receive you in
With all kind love, good thoughts, and reverence.
But Cassius, ever the practical and worldly man, says: