Why is the aside, or speech, that Cassius makes to Brutus in Act III scene 1 before Antony’s speech important in Julius Caesar?
An aside is a speech or remark spoken by a character to the audience or to another character onstage that others onstage are not supposed to hear.
This is a crucial moment in the play, as actually the tragedy that faces the conspirators could have been averted if Brutus had followed the advice that Cassius gives to him in this aside. It is important to note that throughout the play, it is Cassius that is depicted as being canny and a good judge of character. Brutus is presented as being somewhat naive and believing in the power of good intentions. Cassius in this aside recognises the power that Antony has with "the mob," the Roman populace, and also his skill in rhetoric and thus counsels Brutus to not let him address the crowd:
You know not what you do; do not consent
That Antony speak in his funeral.
Know you how much the peaople may be moved
By that which he will utter?
Cassius, himself an able manipulator of others through speech, is aware of the harm and damage Antony could cause through his words, and thus hopes to avert a possible backlash against the conspirators by preventing Antony from speaking. Remember too that Cassius had actually advised that Antony should be murdered along with Caesar, but Brutus, in a response that shows how he underestimated Antony's character, refused. This is yet another example of how Brutus, perhaps through his innocence and belief in goodness, ignores advice that is actually very shrewd and would have helped him greatly if he had paid attention to it.