Interestingly, the relationship between Cassius and Brutus is one that changes throughout the play as the plot develops. Initially, in the famous seduction scene in Act I scene 2, it is Cassius who definitely seems in control and able to manipulate Brutus, exploiting his fears of the potential dangers of Caesar's ever-increasing power for his own purposes. Cassius wants Brutus on the side of the conspirators, as he is a respected and well-renowned person. Therefore, initially, Cassius exploits the naivety and innocence of Brutus for his own purposes, using such lines as the following quote to exaggerate Caesar's power and the danger that might come from that power:
Why, man, he doth bestride the narrow world
Like a Colossus, and we petty men
Walk under his huge legs and peep about
To find ourselves dishonorable graves.
This line deliberately over-emphasises the power of Caesar by comparing him to the Colossus, an ancient wonder of the world, a massive statue that towered over everybody. Against such power, everybody else is made to feel tiny and puny by comparison. With such lines, Cassius is able to win Brutus over to his cause. However, it would be a mistake to assume that Cassius holds the reins of power, as after this, he defers to the judgement of Brutus in a number of key decisions that prove to be fatal for both himself and the conspirators. The first of this is the decision to not kill Mark Antony, which Cassius tries to arrange. He is silent when Brutus insists that Mark Antony is left alive, and again he is silent when Brutus allows Mark Antony to address the public after Caesar's death, both of which prove to be costly mistakes. This is a key area in which the relationship therefore changes, suggesting that Cassius, in spite of his initial power and mastery over Brutus as expressed in the seduction scene in Act I scene 2, actually allows Brutus to take power as the play develops.