In act 3, scene 1, why is the aside from Cassius to Brutus before Antony's speech important?
In Shakespeare's Julius Caesar, the aside when Cassius talks to Brutus about not letting Antony speak to the crowd at Caesar's funeral is important, because it provides an example of Brutus's poor judgment, which is what leads Brutus, the tragic figure in the play, to his fall.
Cassius tells Brutus:
You know not what you do. Do not consent
That Antony speak in his funeral.
Know you how much the people may be moved
By that which he will utter?
But Brutus decides to let Antony speak anyway. Of course, we know what happens. Brutus errs in many ways. First, he underestimates Antony. Second, he overestimates the importance of his announcement to the crowd that Antony speaks only with his permission. Third, he underestimates the fickleness of the Roman crowd. Fourth, he thinks the reasons behind the assassination of the crowd will be evident to and accepted by the crowd without question (and they are, until Antony speaks). Fifth, he doesn't consider the possiblility that Antony will speak with irony, thereby fulfilling his promise to not speak against the conspirators, while still turning the crowd against them (this last one is a bit of a stretch, I know).
Cassius, as it turns out, is a much better judge of people and situations, and a much better decision maker than Brutus. Though Brutus is the noble one of the two, the conspiracy may have succeeded if Cassius were the one making the decisions.