Cassius and Brutus have a changing relationship throughout the play. They are completely different in character. Brutus is honest . Cassius is deceiving and selfish. They also think different about the destiny of Rome.
Initially, it is Cassius who definitely seems in control and able to manipulate Brutus, exploiting his fears of the potential dangers of Caesar's increasing power for his own purposes. Cassius wants Brutus on the side of the conspirators, as he is a respected and well-renowned person.
Brutus misjudges Cassius, and later he spoils the whole conspiracy by trusting Marc Antony. Although Brutus decides to assasinate Caesar rather tan being a true friend to him and speak about the matter, Cassius is a better friend and decides to use Brutus as a sparehead of his plan to kill Caesar. Since Brutus is well-respected, Cassius's conspiracy gains respect as well. Brutus sees Cassius as a true friend and joins the plot.