Why does Cassius fear the conspiracy has been discovered ?
Act 3, Scene 1 of Shakespeare's Julius Caesar takes place near the Capitol at Rome. Caesar himself is present as are those conspiring to kill him. As Caesar walks toward the Senate-House, he is approached by Artemidorus, who wants Caesar immediately to read a petition.
After Cassius tries to move Artemidorus on towards the Capitol, a certain Popilius Lena approaches Cassius and says, "I wish your enterprise to-day may thrive." When Cassius wonders what Popilius is talking about, Popilius says, "Fare you well." Then, Popilius walks toward Caesar.
Cassius is alarmed because he thinks Popilius' words indicate that he knows of the conspiracy to murder Caesar. Cassius appears on the verge of panic, but Brutus calms his fears by pointing out Popilius' behavior:
Cassius, be constant:
Popilius Lena speaks not of our purposes;
For, look, he smiles, and Caesar doth not change.