Cassius’s comments about free will, fate, and being underlings are largely contained in his long speech which begins with: “Why, man, he doth bestride the narrow world / Like a Colossus…” These lines in my edition are 136-137 but may be slightly different in others. The pertinent lines in this speech are :
Men at sometime were masters of their fates.
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings.
Cassius is speaking to Brutus, trying to incite him against Caesar. Cassius keeps referring to Romans of the past, e.g., “Rome, thou hast lost the breed of noble bloods…” He wants to keep reminding Brutus of his noble ancestors who were responsible for founding the republic. Lucius Junius Brutus was famous for expelling the royal family, the Tarquins, out of the city in 509 B.C. and establishing the Roman Republic, which Cassius believes is now threatened by the ascendancy of Julius Caesar backed by his own army and supported by the lower classes of Roman citizens. Cassius is not well liked. He knows he needs someone with prestige and popularity comparable to Caesar’s to lead an effective coup against Caesar.