In Act I scene iii of Julius Caesar, Casca and Cassius meet in the streets of Rome to discuss omens, portents, and Caesar's future in Rome. Both men do not like Caesar and fear that his ambition will lead to the Roman people declaring Caesar emperor of Rome. As a result, Cassius is determined to have Brutus (a long time ally and friend to Caesar) join in a conspiracy to assassinate Caesar.
As revealed in Act I scene ii, Brutus is "with himself at war." He is torn between his care for Caesar and his concern for the Roman Republic. Brutus' ancestors were responsible for driving out the kings and establishing the Republic, so to allow it to be destroyed by one man is like allowing his ancestors to be destroyed as well. He too is fearful of Caesar's ambition and compares him to a "serpent" that must be "killed in the shell." Brutus justifies his decision to assassinate Caesar because it's not that he "loved Caesar less" but "loved Rome more."
Prior to going to Brutus' garden and securing his role in the conspiracy, Cassius and Casca discuss how they can persuade Brutus to join their endeavor. Cassius has previously written false letters that are supposed to be from concerned citizens. The letters praise Brutus and condemn Caesar. Cassius informs Casca of his plan and tells Casca that "three parts" of Brutus are already theirs. He wants to secure the last part. Casca responds,
O, he sits high in all the people's hearts,
And that which would appear offense in us,
His countenance, like richest alchemy,
Will change to virtue and to worthiness (166-169).
Here Casca compares Brutus' countenance to "richest alchemy." Casca is agreeing with Cassius: they need Brutus' support or their plan to assassinate Caesar will fail or be perceived as an act of mutiny. Casca realizes that Brutus is influential; the Roman people admire and trust him. Basically, Brutus has a "magical" effect on the people. If he is part of the conspiracy, then the Roman people will support the conspirators.