After Brutus and his co-conspirators have murdered Caesar in Shakespeare's Julius Caesar, Antony comes to meet with them. He shows intent to deceive the assassins when he tells them that he is not interested in blaming them for what they have done. Cassius has asked Antony if he will be their friend or foe:
Will you be prick'd in number of our friends,
Or shall we on, and not depend on you? (III.i.232-233)
Antony's response is:
Therefore I took your hands…
Friends am I with you all and love you all… (234, 236)
At this point, Brutus contends that even if Antony were Caesar's son, he would have to understand the valid reasons that motivated their assassination of Caesar. Antony does not argue, but asks if he can speak to the people at Caesar's funeral; and though Cassius is against it, Brutus agrees, noting that he will speak first to defend their actions and then let Antony speak.
When everyone leaves but Antony, we see past the pretense he has adopted to where his real motivation lies. He apologizes to the corpse of Caesar for putting on a show for dead Caesar's murderers, but promises that they will be sorry for their deeds—
O, pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers!
Thou art the ruins of the noblest man
That ever lived in the tide of times.
Woe to the hand that shed this costly blood! (274-278)
Then Antony promises that there will be civil war over Caesar's death so horrific, that no one could imagine the suffering and heartache it will bring.
The result of Antony's deception is seen at the funeral when he seems first to agree with everything Brutus says (which joins the people in support of what the assassins have done). However, by the end of Antony's speech (and he is a magnificent orator), he has addressed each point in Brutus' arguments and subtly turned the people against Brutus and his compatriots. Brutus and the others must flee for their lives. This is the start of the civil war Antony promised earlier in his soliloquy.
We also see another side of Antony in terms of one of the other members of the triumvirate. Lepidus is the third member of the triumvirate, along with Antony and Octavius, who rule Rome after Caesar's death. At one point, however, we see the ambitious and vicious side of Antony once again. Octavius notes that Ledipus is a man with a valiant military past. Antony notes that he is easily led. He compares Ledipus' military bravery to that of his own horse. Like a horse, he values Ledipus only for what he can do, and when that is accomplished, like an old horse, he will be "discarded." Antony refers to him as chattel, a piece of "property:"
He must be taught, and train'd, and bid go forth;
A barren-spirited fellow, one that feeds
On objects, arts, and imitations,
Which, out of use and staled by other men,
Begin his fashion. Do not talk of him
But as a property. (IV.i.38-43)
This callous disregard for another human being, as well as his intent to do away with Ledipus when he is no longer useful, shows Antony as a schemer and user, with murderous intent—very different than the noble face he puts forth as Caesar's supporter and then as Caesar's advocate upon his death. Antony shows two very different sides of his personality, and the hidden side does not speak well of his overall character.
Antony didn't change, we just didn't see that side of him yet.
He manipulated the conspiratos, or at least Brutus, into believing he's their friend. The noble Brutus, of course, thought that everyone deserve to be trusted, and let Antony speak. Brutus underestimated Antony's power, which was a huge mistake. If he listened to Cassius, Antony wouldn't have gotten a chance to manipulate the crowd into believing that Brutus is a bad person (which he's not, obviously.)
The side we see of Antony in Act 4, where he degraded Lepidus, show's how much of a jerk he really is. The fact that he killed people so he won't have to give them Caesar's will/money, proves that. Killing Senators, also proves that even more.
I had huge respect for him in Act5 though, because he called Brutus "The noblest Roman of them all"