Act 1, Scene 1
The point of the stage direction is that Flavius and Marullus do not come in with the commoners. Flavius and Marullus are Tribunes of the People, empowered to protect the interest of the Plebians (Commoners). They want to do all they can to derail Caesar's march to power, so when they spot the commoners rushing to see Caesar's triumphal procession, they intercept them and scold them.
In Shakespeare's time it was common knowledge that the Tribunes of the People lost their struggle against the rising power of Julius Caesar and his nephew, Octavian, now known as Caesar Augustus, the first Roman Emperor.