Cassius is a character foil to Brutus. How is this shown in Act 1?

A lot of this has to do with their motivations.  Cassius
is motivated by jealousy.  He is clearly jealous of Caesar's power and feels that Caesar
is weak and doesn't deserve the power he has.  When he talks about having to save Caesar
from drowning when they were younger, it shows that Cassius feels he is better than
Caesar, but that he is always put below Caesar.  When talking about saving Caesar from
drowning, Cassius says that even though he had to save him, "Cassius is A wretched
creature, and must bend his body If Caesar carelessly but nod on him," which again shows
his jealousy and contempt of Caesar's position.

Brutus on
the other hand is motivated by his feelings of loyalty toward Rome.  Although Caesar is
a close friend of Brutus, Brutus can see that Caesar in power would not be the best
thing for Rome.  Brutus is not considering joining the conspiracy for personal gain, but
because he feels it is the best thing for his beloved Rome.