I assume you are refering to Act I scene 3, when Cassius rebukes Casca for the fear that he shows at the signs that are displayed in the night sky. Consider what Cassius says to Casca after Casca has shown his fear and timidity:
You are dull, Casca, and those sparks of life
That should be in a Roman you do want,
Or else you use not. You look pale, and gaze,
And put on fear, and cast yourself in wonder
To see the strange impatience of the heavens...
What is interesting about this rebuke is the way that it shows how Cassius is blind to the signs of nature and what is happening. Casca shows himself to be humble and fearful in the face of such foreboding signs, whereas Cassius sees this as a profound character weakness. Cassius, unlike Casca, sees these signs in the heavens as mere natural phenomena that only fools try to interpret. This points towards a certain level of arrogance and blindness in Cassius, that supports our view of him as a self-seeking manipulator who uses others for his own ends.