When the storm comes in Act I, Scene 3, Casca reacts with great trepidation; Cassius, however, is not intimidated by the bizarre occurrences that they witness.
Since the Chain of Being often plays a role in the structure of Shakespeare's plots and the psychology of his characters, the supernatural plays an essential role in their fates. When Casca witnesses the storm, he tells Cassius,
It is the part [role] of men to fear and tremble
When the most mighty gods by tokens send
Such dreadful heralds to astonish us. (1.3.55-57)
Casca feels that supernatural powers send awe-inspiring announcements to frighten and stun men. He perceives the supernatural happenings of heaven dropping fire, a hand ablaze without burning, a lion loose in the capital, an owl hooting in the marketplace at noon, and men on fire who walk through the streets all as warnings.
In contrast, Cassius interprets these bizarre happenings that they witness as omens that an unnatural state will come if Caesar is crowned as emperor:
Why all these things change from their ordinance
Their natures and preformèd faculties
To monstrous quality—why, you shall find
That heaven hath infused them with these spirits
To make them instruments of fear and warning
Unto some monstrous state. (1.3.67-72)
When Casca asks, "'Tis Caesar that you mean, is it not Cassius?" Cassius explains that Julius Caesar
is a man who does similar things as the heavens have exhibited. He thunders and throws lightning, and he roars just as does the lion in the Capitol. Therefore, Cassius perceives the bizarre happenings as omens that Caesar must be prevented from becoming a tyrant.