As this question is tagged with Act V, do you mean what Antony says about Brutus? If so, Antony is not acting. First of all he realizes that Brutus has killed Caesar for noble reasons. Afterall, Brutus himself states in his oration after Caesar's death that he did love Caesar: "Not that I loved Caesar less, but that I loved Rome more." Antony's oration after Caesar's death is filled with verbal irony in the hopes of turning the people against the conspirators who, besides Brutus, were envious men and were simply out for their personal gain.
When Brutus dies after battles and failures in Act V, Antony lauds Brutus because he realizes that all that Brutus has done has been for patriotic and honest reasons:
This was the noblest Roman of them all./All the conspirators save only he/Did that they did in envy of great Caesar;/ He, only in a general honest thought/And common good to all, made one of them.
Antony explains in the last line that out of an effort for Rome and the common good, Brutus joined the others. Otherwise, this "gentle" man would not have committed the deeds he has.
I don't know what the questioner meant by "acting" in this context, but the above answer by a teacher is 180 degrees off the mark. The entire funeral speech is ironic, in that Marc Antony says exactly the opposite of what he means while intending for his audience to understand his true meaning. I would argue that even the beginning is ironic: "The evil that men do lives after them; the good is oft interred with their bones." You and I and Shakespeare know the opposite is true. When people die it is the good that people tend to remember, not the bad. The old saying goes, "de mortuis nil nisi bonum"-- of the dead speak nothing but good. And after that, the irony in the speech is more obvious. Time and again, Marc Antony cites the good Caesar did and then says Brutus nevertheless said he was bad "and Brutus is an honorable man." This whole speech is one of the best lessons in the use of irony you will ever see.
It is true that later on in the play, Marc Antony calls Brutus "the noblest Roman of them all." But after all, Brutus is dead -- killed in battle against Marc Antony -- and de mortuis nil nisi bonum.