There was a lot of talking and restlessness in Shakespeare's theater before the play began, especially in the pit where the standees were often unruly. Shakespeare's opening scenes typically start with incidents that will capture attention and create silence so that everyone can hear the actors. In Macbeth, for example, the Three Witches are so weird and crazy that everyone is fascinated. In Hamlet there is a suggestion that the guards are in imminent danger and are all frightened. Julius Caesar opens with a big conflict between the tribunes and the plebeians. Theatergoers were probably not accustomed to seeing so many people on the stage when the play had just opened.
Shakespeare does not have to capture audience interest for the rest of the play. That would be impossible. He captures audience interest long enough to silence them and get them involved in the plot. The imbroglio between the tribunes and the commoners not only attracts attention; it illustrates the fundamental problem in the play. There is great unrest in Rome. A lot of people are fed up with democratic government and want a strong man to take over and establish law and order. Julius Caesar is presenting himself as that man. There is a strong possibility that he will be made king. Many aristocrats fear this demagogue because he evidently intends to achieve power by taking money, property, and power from the privileged classes in order to improve the conditions of the have-nots. The conspiracy to assassinate Caesar is essentially intended to preserve the privileges of the rich and powerful.
So Shakespeare's dramatic opening captures immediate interest and also acquaints the audience with the nature of the crisis prevailing at the time. The opening scene ends with dialogue containing a wonderful metaphor which shows that Julius Caesar is at the center of this great crisis.
And drive away the vulgar from the streets;
So do you too, where you perceive them thick.
These growing feathers pluck'd from Caesar's wing
Will make him fly an ordinary pitch,
Who else would soar above the view of men
And keep us all in servile fearfulness.