they saw men engulfed in fire roaming the streets and a night owl screeching at midday.

Casca is frightened by these signs and fears that he and the conspirators are being warned by the gods not to act against Caesar, but Cassius insists that the signs can be interpreted any way men choose to see them. He chooses to believe the gods are not warning the conspirators, but rather warning Rome herself against Caesar. To prove his point, he claims to have walked around in the storm and even bared his chest to the thunder. The gods did not strike him down, so he believes they are on the conspirator's side.

Casca’s purpose in the drama The Tragedy of Julius Caesar by William Shakespeare is as a provider of information.  This is evident in Act I, Scene ii, when he tells Cassius and Brutus what has happened in the arena when Caesar was offered the crown by Marc Antony. 

In Act I, Scene iii, it is the night before the ides of March.   A month has passed and the weather fits the tumult in the hearts of many of the senators who are conspirators.  The storm frightens Casca who is going through the street with his sword drawn to ward off the terrible things that he has seen.

Casca meets Cicero on the street, a famous orator and senator, and describes to him all of the frightening things he has witnessed.    

What has he seen?

• A man has his hand on fire, but it is not burning
• A lion is walking in the streets of Rome
• A nocturnal bird---the owl---is seen at noon in the marketplace.

But wherefore did you so much tempt the heavens?
It is the part of men to fear and tremble
When the most mighty gods by tokens send
Such dreadful heralds to astonish us.

Men on fire are walking in the streets. The earth has been moving and shaking. The winds have pulled old oak trees up from the ground.  The ocean has swelled its waves with rage and foam. The clouds are threatening; and now, the skies drop fire on the streets.

How does Casca interpret these omens? Casca, a superstitious man,  believes that man has not be sacrificing or worshipping the gods enough. To him,  the gods are fighting among themselves and are determined to destroy the city. 

When Casca warns Cassius about the signs from the day’s events, Casca further tells him that senators plan to bestow the crown on Caesar.  Caesar may wear the crown everywhere except in Italy.

This night instills in both of these conspirators a need to carry out the assassination.  Casca, in his fear of the gods, makes the connection that Caesar has displeased the gods and cannot be crowned as the king.

Cassius uses the things that have been seen in the night and the terrible storm as the incentives needed to draw Casca and Brutus into the conspiracy.  Caesar must die on the Ides of March.