The audience can conclude that the crowd of plebeians is very fickle. They change their allegiance on a whim and a word - and they show this clearly in their reactions to the two speeches.
The fickleness of the plebeians has already been seen in Act I, when they celebrate Julius Caesar's return to Rome after defeating Pompey and his sons - despite that fact that, not so long ago, they cheered Pompey in the same way. After the conspirators kill Caesar, their initial reation is to be angry at the conspirators - that is, until Brutus speaks to them.
After Brutus's speech, they are ready to hail Brutus as a hero, shouting that they should "bring him with triumph unto his house" and "give him a statue with his ancestors," even going so far as to call out, "Let him be Caesar (III,ii). Going further, they proclaim, "This Caesar was a tyrant" (III, ii), thus branding him as deserving of what the conspirators have done to him. Brutus has fully convinced them - and thinking this, he departs the scene.
Antony, however, next steps up to the podium, and his words quickly win over the crowd. After he beings speaking, the plebeians just as quickly turn against Brutus. Though Antony repeatedly calls Brutus an "honorable man," he clearly does not truly believe this, and the crowd soon starts to agree with him.
The crowd becomes even more riled up when Antony mentions Caesar's will and the generous bequests he has left to Rome. Soon the plebeians are calling the conspirators traitors, exclaiming "Revenge! About! Seek! Burn! Fire! Kill! Slay! Let not a traitor live!" (III.ii).
Before Antony has even finished speaking, they are calling for blood. And with his final words they take to the streets, burning houses, causing destruction throughout the city, and even killing the innocent poet Cinna because he shares the same name as the conspirator.
All of this shows that the crowd is an unthinking, emotion-filled mob that acts on impulse without thoughtful reason or logic, and without carefully weighing their actions. They are fickle, changeable, and disloyal, following whoever speaks the loudest or most persuasively or whoever promises the most in terms of their own material wealth and happiness.