Explain how Brutus and Cassius act as character foils in their responses to Antony in Act III, Scene 1 of Julius Caesar by William Shakespeare.

It is in Act III, Scene 1 that the differences of
personality between Brutus and Cassius are furthered developed.  After the assassination
of Caesar, Antony tells the conspirators that he is ready to be "cut off," but Brutus
replies graciously,

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O Antony, beg not your death of
us!....

Our arms in strength of malice, and out hearts'Of
brothers' temper, do receive you

With all kind love, good
thoughts, and
reverence.

These words of
Brutus also illustrate his noble nature that is trusting and often does not perceive
what is pratical, unlike Cassius, who is immediately suspicious of
Antony:

I
blame you not for praising Caesar so;

But what compact mean
you to have with us?

Clearly,
the difference between Brutus and Cassius is that of the idealist and the pragmatist. 
For, whether Antony's intentions are good or not, Cassius knows that to be certain of
him is impossible, and the most practical thing to do is to kill Antony.  Pulling Brutus
aside, he mentions the influence upon the crowd that Antony could easily have.  Here
again, the idealistic Brutus demurs, saying "It shall advantage more than do us wrong,"
whereas Brutus replies, "I know not what may fail; I like it
not."