How is Mark Anthony presented in act 3, scene 1 of Julius Caesar?

Antony is portrayed as cunning and clever but also as loyal to Caesar. The first scene of act 3 is when Caesar is assassinated by the conspirators, who then learn that Antony has "fled to his house, amazed" at Caesar's death. Brutus then receives Antony's servant, who comes from his master seeking assurance that the conspirators don't want him dead as well. Once he receives this assurance, he arrives and is so overcome with grief at seeing Caesar's corpse that he actually suggests that Brutus should go ahead and kill him as well. Antony shakes hands with each of the conspirators and says, despite being clearly distraught, that he will support them, and he asks to speak to the crowd assembled outside.

It is at this point that we see Antony's plans. He will not actually support Brutus and Cassius but will conspire against them. In a soliloquy he reveals that he plans to foment a civil war in Rome with the support of Octavian. But he pragmatically determines to gauge the reaction of the Roman crowd to the eulogy he is about to give for Caesar. He will exploit Brutus's decision to allow him to speak to stir up the Roman crowd to avenge Caesar's death.