know you, Casca, I have moved already
Some certain of the noblest-minded Romans
To undergo with me an enterprise
Of honourable-dangerous consequence … (Act 1, Scene 3)

Cassius wants to have as many people involved as possible so that they all are in it together.  He tells Cassius that he has been convincing others, so he must have been at this for a while.  Cassius and the others want to make sure Brutus is on board.  They need his name and reputation to lend legitimacy to the cause.

Casca, for his part, is highly opposed to Caesar.  He says that he would rather die than be Caesar's slave.  Casca and the others think that Caesar is making himself out to be king, and will gather more and more power until he is unstoppable.  Casca is the first to stab Caesar, and his wound is the fatal one.

Cassius and Casca are sneaky in convincing Brutus.  They leave letters for Brutus to find that will make him think that they are from various people, asking him to act against Caesar.

CASSIUS

Be you content: good Cinna, take this paper,
And look you lay it in the praetor's chair,
Where Brutus may but find it … (Act 1, Scene 3)

Brutus finds the letter, and in his speech where he says “it must be by his death,” he seems to talk himself into believing that Caesar is too dangerous to be left alive.  All of Rome is for Brutus acting against Caesar, he thinks.  Brutus believes that killing Caesar is what is best for Rome.

Cassius tells Casca that he already has a lot of supporters for the opposition to Caesar.

Cassius asks Casca to come over so he can talk about the opposition privately.  Once alone, he explains to Casca that he has been trying to gather support for opposition to Caesar, and he has already been successful among some very high-ranking Romans.

Now know you, Casca, I have moved already

Some certain of the noblest-minded Romans

To undergo with me an enterprise

Of honourable-dangerous consequence (Act 1, Scene 3)

This is important because it shows that the forces are mobilizing already, and Brutus, Cassius, and Casca have support among the senators.  Apparently, Cassius has a lot of support.  Caesar is unpopular because he is high and mighty.  He has placed himself above everyone else.  They are worried that he abuses his power, or that he will abuse his power.  Basically, he has not done anything wrong yet, except for being arrogant, but they are afraid that he is too strong and he is going to be too strong to stop before too long.  That is why they have decided to act.

The actions happen in this play very quickly.  In the first act, Shakespeare is already establishing that Caesar considers himself untouchable, like a god, but that he is actually vulnerable both politically and personally (the mention of “falling sickness,” epilepsy, symbolically and actually makes him vulnerable).  He considers himself immortal, like when he doesn’t listen to the soothsayer’s warning.  This kind of thing annoys the senators even more.

While some of the senators, like Brutus, believe that Caesar does not deserve to die, they are also concerned about the consequences of allowing him to go unchecked.  Brutus decides that Caesar is a serpent, and he needs to “kill him in the shell” before he does more damage.

Act I, Scene III of this play is set against a backdrop of thunder, which reflects the conspiratorial nature of the discussions that take place within it between Casca and Cassius. Shortly before Cinna enters the scene—Cassius notes that he recognizes Cinna's footfalls, and that he is a friend—Cassius has just revealed to Casca that he has "moved already" (prevailed upon, or convinced) several "noble-minded" Romans to join him in an "honourable-dangerous" enterprise. The streets are empty on this night, with the "complexion of the element" (the weather) an accurate reflexion of the enterprise the noblemen are about to undertake. Cassius reveals that those who have taken Cassius's side "stay...in Pompey's porch," waiting for Cassius to collect them so that they can embark upon their mission.

Cinna himself is already aware of this information and is glad to hear that Casca has decided to join them in their enterprise.