We learn a great deal about the major players in Julius Caesar in Act I, scene 2.
It is clear that both Brutus and Cassius are concerned over Caesar's growing power. Cassius is sounding Brutus out as to his feelings and he questions how Caesar has gown so strong when he himself has observed his weaknesses. Cassius then compares Brutus to Caesar and shows him that Caesar is no greater. Brutus comes from an old and well respected Patrician family. The Republic is important. Cassius needs Brutus for his plan to work.
Brutus listens but does not commit himself to Cassius. He is cautious and has strong feelings for Caesar as a friend.
Of the two, Cassius is the more ruthless. He has decided that the only solution to Caesar's growing popularity and power is to assassinate him.
For his part, Caesar seems to enjoy the adulation of the mob and Mark Antony seems like a young man who has curried favor with Caesar.