In Shakespeare's Julius Caesar, in Act III, scene i, Popilius says to Cassius that he hopes his "enterprise" or his ambitions will thrive. Cassius is paranoid because of the plans he and co-conspirators have to assassinate Caesar. It may at first seem that Popilius is privy to their secret plans. This also provides the audience with a breathless moment, wondering if the plot is to be denounced and stopped. However, Popilius is not speaking of treachery. It seems he is simply wishing Cassius success in his business that day—whatever it is; as innocent as our parting words, "Have a nice day."
If we search through the scene, we will note that there is a great deal going on! The soothsayer is repeating (to Caesar's annoyance) his "it's-getting-old" warning regarding "the Ides of March." Artemidorus is pushing Caesar to read his letter, which does carry a warning.
There is irony here in several ways. The two men who might save Caesar are the two men he dismisses. It is also noted that it is ironic that Caesar sends Artemidorus away, noting that Caesar will deal with any personal affairs last. Seemingly a noble gesture, he goes on to show he is not at all noble, but really quite full of himself.
Caesar believes that he is a god, not a mere mortal. He also sees himself as the heart and soul of Rome. Consider his response as Metellus Cimber humbles himself before Caesar, asking that he reconsider the emperor's banishment of Cimber's brother.
I must prevent thee, Cimber.
These couchings and these lowly courtesies
Might fire the blood of ordinary men
And turn preordinance and first decree
Into the law of children. Be not fond
To think that Caesar bears such rebel blood
That will be thaw'd from the true quality
With that which melteth fools, I mean sweet words,
Low-crooked court'sies, and base spaniel-fawning. (40-48)
Caesar responds that this kind of humble behavior from Metellus might work on "ordinary men," and might lead them change the laws to those dealing with children. However, it will not sway Caesar's purpose as it would the purpose of fools (which Caesar believes he is not)—showing irony once again in that Caesar is a fool by virtue of his inflated ego—and his failure to recognize that his "friends" are really enemies.
He also says:
Know, Caesar doth not wrong, nor without cause
Will he be satisfied. (52-53)
Caesar says he's never wrong! And again he alienates those around him by comparing himself to the Northern Star, a heavenly body that directs travelers and sailors at night—guiding each to safety...nearly impossible to survive without. He notes that like the star fixed in the heavens, he cannot be moved:
But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament. (66-68)
In this bedlam—as many people seek the attention of Caesar, as well as perhaps an almost palpable feeling of tension from the conspirators—it is easy to imagine that Popilius (who is opposed to Caesar) speaks with knowledge of Cassius' intent; but he never shows this. We can assume that he is simply a part of the melee, delivering solicitous greeting to Cassius as he might any other man present—perhaps vaguely aware of Cassius' discontent—but not suspecting any action based on his unhappiness.
Amid the confusion and rising suspense of the plot development, it is simply Cassius' unique situation which causes him (and the audience) to pause and wonder at Popilius' comment.