Antony acts calmly and in a fairly friendly manner when he encounters Brutus and Cassius right after the assassination of Julius Caesar because he is aware that his own life is in danger, but when he is alone he expresses his true feelings about them in a soliloquy beginning:
O pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers.
Act 3, Scene 1
Then in his famous funeral oration, Antony begins by saying that Brutus and Cassius are "honorable men" but ends by calling them "traitors." (Act 3, Scene 2)
And when he meets them on the battlefield at Philippi, he expresses his full feelings:
Villains, you did not so when your vile daggers
Hacked one another in the sides of Caesar.
You showed your teeth like apes, and fawned like hounds,
And bowed like bondmen, kissing Caesar's feet,
Whilst damned Casca, like a cur, behind,
Struck Caesar on the neck. O you flatterers!
Act 5, Scene 1