Casca is terrified by the violent and ominous storm that occurs in Act 1, Scene 3 ofJulius Caesar, but Cassius does not appear to be the least bit frightened or impressed. He tells Casca that he has been deliberately walking around in the storm
And thus unbraced, Cascaa, as you see,
Have bared my bosom to the thunderstone;
And when the cross blue lightning seemed to open
The breast of heaven, I did present myself
Even in the aim and very flash of it.
Cassius has his faults, but he had plenty of courage and takes pride in displaying it. In addition to courage, he has a strong independent spirit. It is because of his independence that he is so resentful of Caesar and is the leader in the plot to assassinate him. Cassius thinks he is equal to any man and superior to most. It may be that his attitude of superiority has caused many people--including Caesar--to dislike him, and it may be that because Cassius knows he is not well liked that he feels the need to persuade the popular and respected Brutus to become a partner in his plot.
Earlier, in Act 1, Scene 2 Cassius tells Brutus how Caesar once dared him to race acros the Tiber River on a winter day when the river was turbulent.
Accoutred as I was I plunged in,
And bade him follow. So indeed he did.
The torrent roared, and we did buffet it
With lusty sinews, throwing it aside,
And stemming it with hearts of controversy.
But Caesar couldn't make it across, and Cassius saved him from drowning.