Act One contains a huge (and significant) scene between Cassius and Brutus, and the most obvious way that Cassius serves as a foil to Brutus is that he is much more obvious (and bold) in suggesting that the solution to both of their dissatisfaction with Caesar's level of power is assassination.
Brutus only alludes to his feelings:
...Vexed I am
Of late with passions of some difference,
Conceptions only proper to myself.
Yet, Cassius, ever rash and free with his words goes to great lengths to describe both the nature of Brutus' dissatisfaction and his own -- the deification of Caesar.
Cassius is the one that lays out the grievances of the conspirators, though Brutus is the one that will lead them. In this way, he plays the part of the villain of the play, while Brutus remains the Tragic Hero, swept up into the events of the play, but not besmirched by the lust for power and sarcasm that drive Cassius' actions.
Cassius' makes it clear in this scene that he intends to lead a rebellion against Caesar, while, ever patient and thoughtful, Brutus only agrees to consider it:
What you have said
I will consider. What you have to say
I will with patience hear, and find a time
Both meet to hear and answer such high things.
Cassius does round up a group of like-minded conspirators to meet with Brutus at his home in Act II. And, much to Cassius' chagrin, it is Brutus that they look to as their leader, not him.