In Julius Caesar, manipulation seems like a professional sport. Politicians use their rhetorical skills to gain power and to influence large, fickle crowds, and seeming friends lie outright to each other. Persuasion and suggestion are rhetorical skills that play central roles in Julius Caesar, but they also highlight the willingness of individuals in hard times to hear what they want to hear (remind you at all of our own day and age?). It's often unclear whether characters are manipulated by others, or do they simply find in the speech of others an inspiration to do what they might otherwise have been too afraid to do.
Brutus was not persuaded to join the plot by anything Cassius had to say. The idea was already stirring in his brain and he joined based on his own reasoning.
Although Julius Caesar is a master manipulator and exercises impressive crowd control, he's not immune to being manipulated by his so-called friends and political allies.