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 ordinanceTheir natures and preformèd facultiesTo monstrous quality—why, you shall findThat heaven hath infused them with these spiritsTo make them instruments of fear and warningUnto some monstrous state. (1.3.67-72)
Casca is terrified by the storm that opens Act 1 Scene 3. He says “never till tonight, never till now, / Did I go through a tempest dropping fire”(9-10). He imagines there must be “civil strife” in heaven to produce such a storm. He goes on to say he saw other strange sights, all unnatural, such as a slave who held up “his left hand, which did flame and burn…and yet his hand … remained unscorched” (1.3. 16-19). In short, Casca’s fear causes him to think the storm is an omen that nature is warning them about their intent to kill Caesar and his fear of the storm signifies his fear of killing Caesar For Cassius, on the other hand, the storm is exhilarating: “I have walked about the streets, / Submitting me unto the perilous night,/ And….bared my bosom to the thunder-stone” (50-53). When he sees lightning, he puts himself right in its path, daring it to strike him, perhaps, or just to absorb its power and energy (54-55). He does not fear the storm, just as he does not fear killing Caesar. He is confident and perhaps arrogant as well.