There was a lot of nervous tension among the conspirators in Act 3, Scene 1. Popilius Lena, a senator, comes up to Cassius and says:
I wish your enterprise today may thrive.
When Brutus questions Cassius about what Popilius just said, Cassius answers:
He wish'd today our enterprise might thrive.
I fear our purpose is discovered.
When there are so many people involved in such a dangerous conspiracy, there is no way of knowing who else besides the conspirators themselves might know about it. For example, Brutus finally gave in to his wife Portia's pleading and told her precisely, though not onstage, what was going to happen. Who else among the conspirators might have confided their secrets to wives, friends, or others? As Benjamin Franklin once wrote: "Three may keep a secret if two of them are dead." And who else among those wives, friends, and others might have passed on all or parts of the secrets to yet others?
It turns out that Popilius is truly a well-wisher and does not intend to alert Julius Caesar to the assassination plot. Popilius approaches Caesar as both Brutus and Cassius watch them closely. Then Brutus tells Cassius:
Cassius, be constant.
Popilius Lena speaks not of our purposes;
For, look, he smiles, and Caesar doth not change.