What do the lines "Now let it work. Mischief , thou art afoot, Take thou what course thou wilt!" convey?
Antony has spurred the crowd to chaos, and now he will watch what they do.
Antony has just given a speech in which he stirred up the crowd against the conspirators. Remember that before Antony spoke, Brutus spoke. Antony made sure that the crowd knew that Brutus gave him permission to speak, and then he made sure that the crowd knew exactly what happened to Caesar. He points out exactly how they betrayed Caesar.
Throughout his speech, Antony seems to begin by saying that he supports Brutus and the others, but he uses the words “honorable men” sarcastically or ironically. Pretty soon, he has the crowd eating out of his hand. They are ready to condemn Brutus and the others.
Antony shows them Caesar’s will, and Caesar’s body. He tells them that Caesar loved them, and he was going to give them all money. He also points to each stab wound and names a conspirator. In doing so, he whips the crowd into such a frenzy against the conspirators that they begin to decide to tear their houses down and burn them.
Antony is pleased with this. He did not need to do any violence against the conspirators himself. This “mischief” will all be done for him. He never directly tells the crowd to do anything. All he does is incite them to violence by shamelessly playing on their love of Caesar.
Mark Antony was underestimated by Brutus, and here he pays the price. Brutus did not kill him because he thought that Antony was just an arm of Caesar. Then he let Antony speak, because he thought he was just an oaf and a playboy. He underestimated Antony's ambition, and his oratory power.
In one speech, Antony ruined Brutus's stance with the people and any hopes he had of controlling Rome easily and without bloodshed. Now, it is war. Antony, on the other hand, hopes that he can easily step into Caesar's place with this speech. He is forgetting one thing. Caesar has an heir, not just in name, but in cunning and ambition. Antony should not underestimate Octavius.