the role of the plebians.are they weak as flavius and marrullus claim in the opening scene

I think this is accurate, the Plebians are easily manipulated by clever rhetoric and a little psychology of the masses. Brutus' speech, which follows Caesar's death, successfully manipulates the plebeian perspective. By the end of his speech, the crowd is hailing Brutus for killing Caesar, whom they now perceive as a great villain. But, the crowd is easily swayed once again when Antony speaks. Following Brutus' remarks, Antony gives Caesar's eulogy, manipulating the crowd with stories of Caesar's kindness, and sharing the details of Caesar's will, which leaves money to every Roman. At the end of Antony's speech, the crowd is once again supportive of Caesar, mourns his death, and seeks to kill Brutus, Cassius, and the other murderers. The swaying opinions of the plebeians is a device that lets the audience not only judge the orators but their own role in politics as well.