Brutus is a close friend of Julius Caesar, and has his trust. "I love the name of honour more than I fear death." - Act 1 Scene 2. This shows that Brutus is an honourable and patriotic man, who would willingly sacrifice his life if it benefits Rome. In this case, he conspires against his friend Caesar and risks his life doing so for he believes that Caesar has grown arrogant and may turn corrupt from power. This is shown in "that lowliness is young ambition's ladder, whereto the climber-upward turns his face; but when he once attains the upmost round. He then unto the ladder turns his back." - Act 2 Scene 1.
Cassius is a high class person who has an enormous greed for power. He is so jealous of Caesar for being the sole leader of what he thought was the world then that he wants to kill him and hopefully he will get more power. This is shown in "Now is it Rome indeed and room enough, when there is in it but one only man?" - Act 1 Scene 2.
The theme of power is an important one in Julius Caesar. Power is what led Brutus to fear Caesar would turn corrupt and therefore resulted in him conspiring against Caesar. It is also this love of power from Cassius that caused him to plot against Caesar.
The theme of loyalty is shown by Brutus. "It must be by his death: and for my part,
I know no personal cause to spurn at him, but for the general." Act 2 Scene 1. Despite being a close friend to Caesar, he kills him due to greater loyalty to Rome than his loyalty to Caesar.
Corruption is shown by Caesar and Cassius, both on account of love of power. "Have I in conquest stretch'd mine arm so far, to be afraid to tell graybeards the truth?" - Act 2 Scene 2. Caesar is arrogant and disrespects even people as high as the