There was a great deal that happened between the time that Antony turned the Roman people against the conspirators and the battle at Philippi in which Brutus and Cassius were defeated and lost their lives. It took both Brutus and Cassius a long time to raise armies in Greece and elsewhere. There was great unrest and bloodshed all over Italy. Shakespeare could not dramatize all this in his play, but he has Antony describe it in capsule form in his soliloquy ending with the marvelous image of "carrion men groaning for burial." This soliloquy not only reveals Antony's true thoughts and feelings, but serves as a summary of history in the form of a prophesy. Shakespeare had a lot of history to work with when he wrote Julius Caesar. He probably felt it was more important to have Antony compress some history in his soliloquy than it was for him to reveal his own thoughts and feelings about the death of his friend Caesar and about the men who had conspired to assassinate him. Antony's famous funeral speech might have been even more effective if the audience hadn't been informed in advance that Antony really hated the conspirators although he had pretended to be won over to their cause.
Shakespeare gave enough hints about Antony's character throughout the play that indicated that there was more to Mark Antony than any of the conspirators (especially Brutus) gave him credit for. Actually, Cassius had him pegged pretty well, but Brutus wouldn't listen to him. This comes back to Brutus when the army's face off in Act V and Cassius remarks to Brutus that if things had gone his way, the parlay with Antony and Octavius would never have taken place.
Antony's skill as a performer is referred to in Act I. In Act II, Brutus seriously underestimates him when he tells Cassius that it is not necessary to assassinate Antony as well as Caesar because Antony is like one of Caesar's limbs, and without the head, the limbs will die, too. Brutus actually believes that Antony will just shrivel up and go away no matter how much Cassius tried to convince him otherwise. Finally, when Antony is shaking bloody hands with the conspirators, I knew that he was just biding his time, formulating a plan in his mind. My suspicions were confirmed with Antony's soliloquy over Caesar's body with his words, "Oh pardon me, though bleeding piece of earth/That I am meek and gentle with these butchers!". From then on, it was just a matter of sitting back and waiting to see what Antony had up his sleeve.
Antony does a fantastic job of leaving himself a way out. He tells the conspirators that he will support them IF they give a good reason why Caesar needed to die. At this point, his motives in talking to the conspirators were starting to be exposed. For that reason, his quick change of mind once alone didn't suprise me either. Antony didn't waste any time using violent and harsh language in referring both to the conspirators and to the act of killing JC. I'd expect that type of reaction from a good friend.
It didn't surprise me too much simply because I understood what he was doing and what his fears were. If he were to have verbally defended Caesar very much, it is quite likely that the conspirators would have killed him, too. They were all standing there, with Caesar's blood dripping from their hands, so he knew that he had to be careful and clever in what he said. He probably also knew that if he lied and said, "Great job, guys, wish you had asked me along," they would have seen right through him and known he was lying (I also don't believe his honor would have allowed him to lie like that).
Antony shares his thoughts with the audience in his soliloquy of Act 3, scene 1, so that we know where he was coming from all along in dealing with the conspirators:
"Oh, pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers!"
We know instantly, then, that he was not giving his true feelings and thoughts to the conspirators, and that he would rather have been able to tell them what he really thought of their conspiracy. He asks Caesar to forgive him for being "meek and gentle," and then goes on to prophesy what will happen in Italy because of this horrible, unnatural murder of Caesar.
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