Why is Mark Antony meek and gentle?

Mark Antony doesn't really have much choice in the matter. His best friend has just been brutally murdered, and if he challenges his assassins in any way, then it is an absolute certainty he will suffer the same fate. At the same time, Mark Antony is biding his time. He wants to avenge Caesar's murder, but he knows that it is too soon after the bloody deed for him to spring into action. For now, he wants to size up his enemies and see what they are made of. Then, when the moment is right, he will exact revenge upon them.
And that is exactly what he does. Mark Antony is a highly intelligent man and certainly much more skillful at the black arts of Roman power politics than Caesar's killers. In carrying out the assassination, they appear motivated by little more than a vague notion of republican liberty. As such, they have no idea as to what precise arrangements will replace the reign of Caesar. They hate the common people of Rome but are at the same time deeply afraid of them. Mark Antony understands this, which is why he cunningly rouses the plebs during his famous funeral speech. After this pivotal moment in the play, no one will ever accuse of him of being "meek and gentle" again.