In Julius Caesar, compare the actions of Antony to those of Cassius and Brutus. What do they reveal about his character?

When we meet Antony for the first time, we discover, significantly, that he is with Caesar and is about ready to start his run on the course during the Lupercalian Festival, which is a traditional celebration of purification and fertility. The fact that he is so close to Caesar indicates the depth of the relationship between the two and his participation in running the course informs us that he has a vivacious spirit and is athletic. This much is alluded to by Brutus in his conversation with Cassius:

I do lack some part
Of that quick spirit that is in Antony.

Brutus is in a glum mood and appears unfriendly, a fact which Cassius quickly latches on to. Cassius discovers that Brutus is displeased with himself and is fighting an internal battle. Brutus also makes him aware later that he is concerned about Caesar becoming ruler of Rome. He states that he would sacrifice his life for the honour of Rome as can be gauged from his response to Cassius' query about whether he would not have Caesar crowned emperor:

I would not, Cassius; yet I love him well.
But wherefore do you hold me here so long?
What is it that you would impart to me?
If it be aught toward the general good,
Set honour in one eye and death i' the other,
And I will look on both indifferently,
or let the gods so speed me as I love
The name of honour more than I fear death.

Cassius comes across as someone not to be trusted, as Caesar himself observes in speaking to Antony:

...Yond Cassius has a lean and hungry look;
He thinks too much: such men are dangerous. 

Caesar's mistrust is not unfounded, for Cassius is the chief conspirator in the plot to assassinate him. Cassius is manipulative and will use any means to gain support for his malicious scheme. He does so with Brutus, for example, who he at first flatters and then tells about Caesar's weakness and how they are more than his equals and should not allow him to subjugate them.

Cassius is driven by ambition and personal gain, whilst Brutus' displays an honourable desire to save Rome from perdition - he wishes to act for the general good. Antony, on the other hand, shows an absolute love and loyalty to Caesar. Since both Cassius and Brutus are aware of his allegiance to the general, they do not approach him for support in their pernicious plot. Brutus describes him as 'a limb of Caesar.'

It is also clear that Brutus believes that Antony is not politically ambitious for he tells Cassius, after the same's suggestion that they murder him too:

And for Mark Antony, think not of him;
For he can do no more than Caesar's arm
When Caesar's head is off.

When Cassius mentions that he fears Antony because he bears an 'ingrafted love' for Caesar, Brutus replies:

Alas, good Cassius, do not think of him:
If he love Caesar, all that he can do
Is to himself, take thought and die for Caesar:
And that were much he should; for he is given
To sports, to wildness and much company.

It is clear that Brutus sees Antony as harmless and weak, one who would be unable to function without his leader, a person who is more interested in socialising and having fun. It is only later that he realises that he completely misjudged Antony.

This poor judgement is most pertinently displayed once the conspirators have assassinated Caesar in the senate house. Antony flees but returns later and utters a passionate speech over the brutally slain body. He asks the conspirators why they believed Caesar was dangerous and Brutus answers:

Or else were this a savage spectacle:
Our reasons are so full of good regard
That were you, Antony, the son of Caesar,
You should be satisfied.

Antony accepts what Brutus tells him and then requests that he be allowed to carry Caesar's corpse to the marketplace and there address the crowd. The naive Brutus accedes but Cassius is upset about him allowing Antony this honour. 

After Brutus has spoken and given reasons for Caesar's murder, he asks the crowd, who has accepted his explanation, to not leave and listen to Antony speak. It is in his brilliant oration that Antony's true nature is displayed. He manipulates the crowd, who have been in support of the conspirators after Brutus' speech, to such an extent that they become restless and unruly.

Antony stays the citizens several times and works them into a frenzy until they become like caged animals, hungry for blood. Once Antony realises that they have become unstoppable, he lets them go. The vindictive crowd surges through the streets of Rome, seeking the conspirators who have, by this time, left in a hurry.

The wily Antony has achieved his purpose and comments:

Now let it work. Mischief, thou art afoot,
Take thou what course thou wilt!

He has purposefully driven the crowd to mutiny and in so doing, has achieved part of his vengeance. With the conspirators driven out of Rome, he can, with his forces and Octavius' support, attack and finally destroy his enemies who would be vulnerable out in the open.   

Clearly, Antony is a man of integrity and honour, one who truly acted in the best interests of not only his beloved Rome but also in the interests of his Caesar, unlike Cassius, who was malicious and self-centred and Brutus, who was somewhat misguided and naive.