One important quotation comes at the beginning of the scene when Macbeth asks the murderer: "Is he dispatch'd?" Is Banquo dead?
When Macbeth sees the ghost of Banquo sitting in his chair at dinner, he is shocked and starts babbling about not being the one who "did it": "Thou canst not say I did it:...." Lady Macbeth understands that he is talking about having murdered his friend Banquo. She tells him:
O proper stuff!
This is the very painting of your fear:
This is the air-drawn dagger which, you said,
Led you to Duncan. O, these flaws, and starts,--
Impostors to true fear,--would well become
A woman's story at a winter's fire,
Authoriz'd by her grandam. Shame itself!
Why do you make such faces? When all's done,
You look but on a stool.
In other words, it's his guilty conscience talking, and he's only looking at an empty seat.
Later in the scene, Macbeth says, "they say, blood will have blood," meaning that the spirit of the murdered person cannot rest until it has been avenged. He also says, "I am in blood/Stepp'd in so far that, should I wade no more,/Returning were as tedious as go o'er:..." He is knee-deep in bloody murder.
Macbeth, speaking to the murderer, says: "But now I am cabin'd, cribb'd, confined, bound in/To saucy doubts and fears. But Banquo's safe?"
By this, he is commenting on how he is in a "fit" that Fleance escaped but restating that Banquo is "dispatch'd": "Is he dispatch'd?" The irony here is that he uses the word "safe." Obviously, Banquo is and isn't safe--he's in heaven, with Duncan, safely away from all evils of the world; and he is dead and bloody in a ditch...definitely not safe.
Macbeth also says, "There the grown serpent lies; the worm that's fled/Hath nature that in time will venom breed,/No teeth for the present." Here, he is commenting on Banquo's death: being a grown and most dangerous serpent, Banquo is now no longer a detriment to Macbeth. Fleance, the worm, has escaped. Macbeth is not presently worried about him since he is not in adulthood and not considered as dangerous as his father is, although he will be in the future.
There are several significant quotes because this is the scene where first, Macbeth learns that the murderers were successful in killing Banquo but unsuccessful in killing Fleance, who escaped (ll. 20-23 and l.24). Then, as Macbeth is preparing to sit down in his seat at a formal dinner attended by several of this thanes, he sees Banquo's ghost, not once, but twice (ll. 61-62). Lady Macbeth has to make excuses for Macbeth's bizarre behavior (ll.65-69) as she chastises him for his actions (ll.73-75). After everyone has gone, Macbeth asks where Macduff was and then says he'll check with his spies that he keeps at Macduff's to see what's up with Macduff. Lady Macbeth tells her husband that his problem is lack of sleep (l. 166). Macbeth then says he'll seek out the witches again the next day to find out more prophecies.