Certainly, much of the action of Act I is intrinsic to the essential to the plot of Julius Caesar. First of all, Caesar returns from a campaign in which he has killed Pompey's sons, an act that raises questions in the minds of the Romans since Pompey was an ally of Rome. The tribunes remind the citizens of the support and love that Rome gave Pompey, and Flavius and Marullus drive the crowd from the streets and pull down banners that honor Caesar in their anger. Then, Caesar is told to beware the Ides of March by a soothsayer, but he arrogantly dismisses him as "a dreamer."
Early in this play, the subjectivity of the characters causes them to interpret events according to their judgments as do Flavius and Marullus. Cassius, who is envious, certainly perceives Caesar as a tyrant under whose yoke he and Brutus "groan," telling Brutus that Caesar "Is now become a god" and he but "A wretched creature" (1.2.122-123). Moreover, he compares Caesar to a Colossus under whose legs he and the other "petty men" must walk. Casca then relates how Caesar and Marc Antony have paraded through the streets of Rome, and Antony has offered him a crown, a coronet, that three times Caesar refused, but he did so reluctantly, suggesting that Caesar truly desires to become emperor of Rome.
At the end of Act I in Scene 3, Cassius hands Cinna a collection of letters that he has forged [these letters are mentioned in his soliloquy]. By putting these in the hands of Brutus, Cassius hopes to sway the noble Brutus who loves Rome to join the plot to assassinate Caesar. For, because of his nature, when Brutus reads these letters, he will feel compelled to act in the cause of preserving the Romans from tyranny, especially after what Cassius and Brutus have already related to him regarding Caesar.