What is the purpose of the interchange between Murellus and the commoners?
The encounter between the Tribunes and the Commoners in the opening scene of Shakespeare'sJulius Caesaris a quick way of gaining the audience's attention and informing them that Julius Caesar is becoming dangerously prominent in Roman politics. This establishes the approximate time and what the play is about.
Shakespeare also had a practical purpose for creating this scene. He needed to employ a number of extras for the crucial scene in which Mark Antony turns the mob against the conspirators with his brilliant funeral speech. Shakespeare evidently wanted to get additional use out of these extras, probably mostly men recruited right off the street, so he invented a couple of additional scenes for them. The first is the one in which the two Tribunes rebuke the Commoners for honoring Caesar and send them home. The one man who plays the Cobbler would undoubtedly be a professional actor and a regular member of Shakespeare's company. He is the only Commoner who has a speaking part in that scene.
These extras will appear again in Act III, Scene 2 as members of the mob listening to Brutus, then to Antony, and finally starting a riot. This was Shakespeare's main purpose for assembling them. He had to have a mob for this vital scene. This is part of real history.
The extras next appear in a sort of "bonus" scene (III.3) in which they encounter Cinna the poet and tear him to pieces because he bears the same name as one of the conspirators. This scene may be used to show Shakespeare's contempt for the lower class, but it mainly serves to get some additional work out of the extras who are being paid whether they are acting or just loafing around.
No doubt Flavius and Murellus returned to life after having been "put to silence for pulling scarves off Caesar's images." Shakespeare would have found uses for these two actors in other roles. They probably were among the crowd of robed men who surrounded Caesar and assassinated him. Shakespeare was not only a playwright but a director, producer, and co-owner of the theater. It was a money-making enterprise, and the less spent on labor the more profit there would be for the entrepreneurs.