During Caesar's assassination the reader does not really know much about the character of Marc Antony. We simply know that he is the "right-hand man" of Julius Caesar and that he, as far as we know, is true and honorable to Julius Caesar. Furthermore, some of the other characters make reference to Anonty as being a party boy. When the conspirators are plotting the assassination, Brutus and a couple others say they should not kill him because he is nothing without Caesar and pretty much say Antony cares too much about having a good time to get too worked up.
However, once Caear is assassinated we see a new side of him. He is manipulative. He is clever and cunning. He tricks Brutus into letting him speak at Caesar's funeral where he proves himself to be very persusive; he has the abilty to "stir men's blood."
But yet again at the beginning of Acts 4 and 5 we see a new side of him. At the beginning at the Act 4, he is planning with Octavius and Lepidus to try to take the money that Caesar left the Roman citizens. Here we see him being very greedy.
Finally at the beginning of Act 5 we see him trying to act should be the man in charge, not Octavius. He was Caesar's right hand man after all. Even though Octavius clearly overtakes him, we here see Antony being a bit power hungry.