In Julius Caesar, Act I is important for laying the groundwork for everything else that will happen in the play. The first scene opens with two tribunes, Marullus and Flavius. Basically, the role of these men is to keep order in the streets, something like policemen. They notice that many people are out and about in the streets on a work day, when they likely should be hard at work at their jobs. This seems suspect to Marullus and Flavius.

     Marullus approaches one group of commoners and asks what going on. In particular, he addresses a shoemaker, or cobbler. A humorous exchange takes place between the two, as a misunderstanding over the word "cobbler" arises. A cobbler is a shoemaker but, in Shakespeare's day, it could also mean someone who tended to botch their work. After Marullus and the cobbler sort this out, the cobbler finally reveals that the people are out in the streets to celebrate a recent victor of Julius Caesar. Caesar, apparently, has defeated another leader named Pompey in battle.

     This news enrages Marullus. He scolds the commoners because it wasn't long ago that they were celebrating Pompey. They seem unaware of where their true loyalties do or should lie. After the commoners have departed, Marullus and Flavius go through the streets of Rome, taking down any decorations that have been put up in celebration of Caesar's victory.

     In scene II, the focus of the play shifts over to Caesar. The Feast of Lupercal is currently taking place and Caesar, his followers, and his wife have gathered to watch the festivities. In particular, Caesar's friend Antony is scheduled to run in a race. Caesar tells Antony to touch his wife, Calpurnia, as he runs by because according to local superstition, it is good luck for a runner in the race to touch a woman who wants to have children. It is interesting that Caesar, who has just come to power, is already concerned about cementing his power through an heir.

     After this, a man-a soothsayer-approaches Caesar to tell him to "beware the Ides of March," or March 15. Caesar brushes off this warning. However, this is some important foreshadowing-a hint at what is to come later in the play.

     Soon, the scene pans over to two other men, Marcus Brutus and Caius Cassius. Brutus is a senator, a long-time friend of Caesar, and a well-respected leader in Rome. Cassius is a general and has known Caesar for many years. A long conversation between the two men ensues.

     In particular, Cassius has noticed that Brutus has not been acting like himself, and he tries to convince Brutus to tell him what exactly has been bothering him. They hear shouting at one point and Brutus says that he is afraid the people have made Caesar king. This is exactly the clue that Cassius needs, and he rightly concludes that Brutus is afraid that Caesar has gained too much power.

     Cassius then begins a series of speeches in which he works to turn Brutus against Caesar. He tells Brutus that Caesar is actually a weak man, and that the weight of Brutus' name could stir people to follow him just as surely as Caesar's.

     Caesar and his followers return, and the scene shifts back to them for a brief exchange. Caesar tells Antony that he thinks Cassius looks like a dangerous man who thinks too much. Ever confident, however, Caesar says he is just making an observation and that he himself does not fear Cassius.

     The scene shifts again to Cassius and Brutus, who have pulled a man named Casca aside to ask about the shouting. Casca tells them that Caesar was offered a crown three times but that he refused the crown each time. Following his refusal, Caesar fell down and had a seizure. However, the crowd seemed to love him no less after this public display of physical weakness.

     After this, Casca departs and Brutus soon after him, leaving Cassius alone on stage. Cassius gives a soliloquy, a speech a character gives on stage when they are alone and which is intended to reveal their inner thoughts. He reveals that he knows he will have to work hard at winning Brutus over. He wants Brutus to see Caesar as a potential threat and join him in his plans to take down Caesar. Cassius plans to write several letters and deliver them to Brutus anonymously. These letters will seem to come from the public, asking Brutus to do something about the too-powerful Caesar. Cassius knows that, more than anything else, Brutus will do what he thinks is best for the general public.

     In the final scene of the Act, Cassius and Casca meet as strange things are happening in Rome. Casca reports that a storm has been "dropping fire," and that he saw a man on fire, even though he didn't burn. An owl-a bird of night-was seen shrieking at noon, while a lion roamed the streets. Casca thinks these are all signs that the gods must be angry at something. Cassius, however, thinks that the gods are actually upset about Caesar's rise to power.

     Casca tells Cassius that the senators are planning to make Caesar king. Cassius begins a tirade against Caesar, basically asserting that he has become much too powerful. Casca agrees with him. Cassius reveals to Casca that he has already convinced a number of powerful Romans to form a conspiracy against Caesar.

     Another conspirator enters the scene, a man named Cinna. Cassius gives Cinna the false letters he has written and instructs him to deliver these to places where Brutus will surely find them. Cassius believes that it is very important that they gain Brutus to the side of the conspiracy because Brutus is a well-known and respected leader in Rome. Having him on their side will give legitimacy to the conspiracy.

     In this section of the play, the central conflict of the play is revealed. With Caesar's quick rise to power, he centers himself as a polarizing figure. The people of Rome clearly support him, despite their previous support of Pompey, whom he defeated in battle. However, as Cassius says, this could be because the people of Rome are weak, like sheep, while Caesar is a predatory wolf. Many public figures, such as Cassius, are openly opposed to Caesar's reign.

     At the center of this conflict is Brutus, and this conflict will be a focal point for much of the play. Brutus is a noble person and admirable politician who always has the public's best interest at heart. Like Cassius, he is not supportive of Caesar's quick rise to power, nor the amount of power he has been given. However, Brutus is torn because Caesar is a long-time friend and one who trusts him completely. He is faced with two evils: should he let Caesar rule and possibly become a tyrant? Or should he participate in a conspiracy that would stop Caesar from becoming too powerful?

     Cassius is in many ways a foil, or contrast, to Brutus as a character. Whereas Brutus is motivated only by what is right, Cassius' motives seem more selfish. He seems to be driven by greed or jealousy and is perfectly willing to manipulate Brutus in whatever way possible in order to get him to join the conspiracy. He even mentions in the first Act that though Brutus is a noble person, no person is so steadfast in their beliefs that they cannot be swayed. Thus, he expresses his belief that he can easily win Brutus over to the side of the conspiracy.