One of the central themes of the play is the fickleness of the plebeians. From the beginning to the end, the plebeians have no lasting convictions. Through a few good speeches, they are moved one way or another. They are mindless.
We get a sense of this right from the beginning of the play. In the opening of the play, there are several commoners standing around trying to get a glimpse of Caesar. They are shouting for him to praise him. When Marcullus and Flavius see this, they question the commoners. Why are they praising Caesar? Why are they so excited?
Marcullus points out that only a little while ago, they were praising Pompey, the enemy of Caesar. What happened? The point is that the crowd is fickle. Marcullus' quotes says it all:
Wherefore rejoice? What conquest brings he home? What tributaries follow him to Rome, To grace in captive bonds his chariot-wheels? You blocks, you stones, you worse than senseless things! O you hard hearts, you cruel men of Rome, Knew you not Pompey? Many a time and oft Have you climb'd up to walls and battlements, To towers and windows, yea, to chimney-tops, Your infants in your arms, and there have sat The livelong day, with patient expectation, To see great Pompey pass the streets of Rome: