Shakespeare uses his first acts to introduce characters and the central conflict of his plays. In Julius Caesar, Act 1, the audience not only witnesses the tension between former supporters of Pompey and Caesar's minions, but they also meet Cassius who was a Pompey supporter and who holds a personal grudge against Caesar. In Cassius's conversation with Brutus, he tirelessly advocates assassinating Caesar. All of this occurs while Caesar celebrates with the people and watches a race.
When the scene switches to Caesar and Antony, Caesar mentions that Cassius has "a lean and hungry look" and that he would rather be surrounded by fat men. Through Caesar's comments, Shakespeare establishes that neither man trusts the other. Cassius believes Caesar to be overly ambitious and corrupt. Caesar knows that Cassius is a schemer who is "hungry" for control and who cannot be satisfied.