How does the allusion to Cassius in Fahrenheit 451 add meaning to the plot?
After Montag's house is burned, Montag takes the hose and turns it toward Captain Beatty. With his life in imminent danger, Beatty taunts Montag with an illusion to Cassuis:
There is no terror, Cassius, in your threats, for I am arm'd so strong in honesty that they pass me by as the idle wind, which I respect not.
This reference comes from Act IV, Scene III of Shakespeare's Julius Caesar. By taking on the role of Brutus, Beatty is portraying himself as the tragic hero: he has tried to help Montag, tried to persuade him that books are pointless and cannot lead to happiness, and, in return for his advice, Montag now threatens his life.
The threat on his life, however, does not hold any "terror" for Beatty because he believes in the system. Even if Montag kills him, a replacement captain will soon follow. In other words, Beatty is not afraid of dying because he knows that his murder is not enough to overthrow the firemen and revolutionise society.
In this understanding, then, the allusion to Cassius represents Montag's powerlessness and his social isolation. This is, perhaps, why Montag fires the hose at Beatty and kills him: he is demonstrating his commitment to changing society and saying, in a most violent manner, that nothing will stand in his way. He is prepared to do whatever it takes to change society's attitude to books and to remove all traces of censorship.
(For more information on Cassius and Brutus, please see the second reference link provided.)