William Shakespeare uses Act I to establish the personalities of many of the important characters, combined with the mood of political conflict mixed with superstition, and provide the basic plot structure—that Julius Caesar has gained power and will be assassinated.

It is interesting that he waits until Scene 2 to put Caesar himself on stage, and even then we hear about him more than from him. The dialogue in Scene 1 has already established the popular controversy about his rulership. After Caesar passes in his procession, we hear the conspirators arguing about their plan. This is foreshadowing to the rifts that will follow the assassination.

Shakespeare clearly establishes two contrasting personalities in the primary traitors. As they jockey for the upper hand, their relative strengths and weaknesses emerge. We hear Brutus speaking of his concern for the welfare of Rome, and Cassius' efforts to use that as the basis for his manipulations. The impression that each one makes is tightly tied in to their interactions, as the playwright hints that they will bring about their own downfall as much as that of Caesar.

I would go the other way and say I was impressed with the importance that Brutus carries about him.  Even though we really aren't given much about Brutus, it is apparent that he is a man of great worth.  Not only does Cassius spend the entire first act trying to convince him to join the conspiracy, but Casca also knows he's important enough to tell what happened at the Capitol that day.  We are able to see Brutus as an extremely important asset without knowing exactly what it is that made him so important.

I suppose Cassius made the biggest impression on me, simply because he's such a smarmy jerk.  He's like an oily, used-car salesman (nothing personal to any used-car salesmen out there!) that knows exactly what to say and who to say it to.  A politician...that's what he is!  Caesar describes him perfectly when he says he "has a lean and hungry look." (1.2)