How does Shakespeare portray the commoners, (Plebians) in Act 3?
In this act, the commoners are the audience for both Brutus and Mark Antony. This is after the death of their leader, Julius Caesar, whom they loved.
They are angry at first, but listen to Brutus who gains their trust. He tells them the reasons the conspirators felt they had to protect Rome by killing Caesar. They cheer him on, and then refuse to stay when Mark Antony comes to speak. It is Brutus who convinces them to stay and listen to Antony whom Brutus tells the crowd, has his permission to address the audience.
When Antony speaks, he proves his skill as an orator. He takes the previously loyal to Brutus crowd and turns them on a dime. He shows them the dead body of Caesar, and turns them against Brutus by calling Brutus an honorable man...many times. The tone of his speech, rather than the words of his speech, suggest that Brutus and the conspirators were anything but honorable and noble. Antony leaves the crowd angry and ready for revenge. They set off and kill Cinna the Poet in a case of mistaken identity.
Therefore, Shakespeare presents the crowd as easily changeable and wavering depending on the skill of the speaker. The crowd is essentially as powerful as its leader/speaker, and capable of great anger and vengeance. They are not to be taken for granted (as Brutus did) and easily molded into what one wants from them (as Antony did).