Shakespeare always had to keep his audience in mind. They would be seeing a number of different men dressed in costumes supposed to look like those actually worn by ancient Romans. The audience would not, of course, recognize one as Brutus, one as Cassius, one as Caesar, etc. An important logistical problem for the playwright was simply distinguishing the various actors by names, characters, motives, sympathies, and alliances--just in order to establlish their identities. It should be noted that these characters are always calling each other by name, although this is not natural in ordinary conversation. For example, when Brutus and Cassius hold their initial conversation in Act 1, Scene 2, there is an obvious effort to enable the audience to tell them apart.
I am not gamesome: I do lack some part
Of that quick spirit that is in Antony.
Let me not hinder, Cassius, your desires;
I'll leave you.
Brutus, I do observe you now of late:
I have not from your eyes that gentleness
And show of love as I was wont to have:
You bear too stubborn and too strange a hand
Over your friend that loves you.
Be not deceived: if I have veil'd my look,
I turn the trouble of my countenance
Merely upon myself. Vexed I am
Of late with passions of some difference,
Conceptions only proper to myself,
Which give some soil perhaps to my behaviors;
But let not therefore my good friends be grieved--
Among which number, Cassius, be you one--
Nor construe any further my neglect,
Than that poor Brutus, with himself at war,
Forgets the shows of love to other men.
Then, Brutus, I have much mistook your passion;
By means whereof this breast of mine hath buried
Thoughts of great value, worthy cogitations.
Tell me, good Brutus, can you see your face?
No, Cassius; for the eye sees not itself,
But by reflection, by some other things.
This sort of name-calling, or "tagging," continues throughout the play. Not only does Brutus call Cassius Cassius, but he even refers to himself as Brutus. They also refer to others by name, including Caesar, Antony, and Casca. The audience gradually remembers that Cassius is not Caesar because Cassius is referring to some other actor as Caesar, and Caesar is not Antony, and so on. There are only two women in the play, Calpurnia and Portia, and they seem to have been included mainly for some contrast to all the men in their robes and togas. The only male character who may be distinguished by his costume is probably Caesar, because his robe will be important later on.
Shakespeare portrays Brutus as noble and public-spirited in contrast to Cassius, who is selfish and cunning. Both men are members of the old aristocracy and quite correctly fear that Caesar intends to become sole ruler of Rome. Brutus is a deep thinker and guarded in his speech. Cassius is emotional and garrulous. Brutus is an introvert; Cassius is an extravert. Brutus is a philosopher; Cassius is a schemer and manipulator who cares about money and not about truth or ideals. The best comparison between the two men is given by Cassius after his conversation with Brutus in Act 1, Scene 2. (Note how many times he mentions names, including his own.)
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.