In Act 3, Scene 4, Macbeth is invited to sit at the banquet table and sees Banquo occupying his place. Macbeth asks, "Which of you have done this?" What specifically does he think some person or persons have done?
Macbeth must suspect that there has been collusion involving at least two of the men in attendance and possibly many or all of them. Note that he asks, "Which of you have done this?" and not "Which of you has done this?" It is true that none of them can see what Macbeth sees, but everybody in the audience can see Banquo sittiing there at the table. Shakespeare obviously had the actor who played Banquo sitting at the table with his hair covered in blood and possibly with some other makeup to have him recognizable but ghostly, such as white powder on his face. My question should have been: Does Macbeth think the lords responsible (1) have used Banquo's body? (2) have made a dummy to look like Banquo? (3) have dressed up someone who resembles Banquo to serve as a substitute? (4) or is it possible that Macbeth thinks some of his guests have somehow managed to bring Banquo's ghost to the banquet? (5) or finally, is it possible that Macbeth thinks he is misinformed by the murderers, that Banquo is still alive and participating in a plot against him by pretending to be his own ghost??? I am only asking about what Macbeth thinks has been done, since the audience has no doubt that this is really Banquo's ghost and that ghosts can go anywhere they like. When Macbeth addresses the ghost directly, he may have forgotten his suspicions and realized that this is Banquo himself who has appeared as a ghost on his own volition.
Macbeth's question "Which of you have done this?" shows that he suspects everybody of conspiring against him although he has been king for only a short time. Since he decided to kill Duncan, he wouldn't be the least bit surprised to find that others are thinking of killing him. And in fact there are probably a few who actually are toying with the idea, just as Macbeth was toying with the idea even before he encountered the three witches. This is not a friendly gathering but a gathering of rivals and enemies. Macbeth would not be paranoid if he thought some of the guests were plotting to assassinate him. As far as Macbeth knows, the assembled lords might be planning to kill him right then and there.
We (in the audience) know that Banquo is really dead because we saw him being stabbed to death--and probably that was Shakespeare's reason for staging the actual murder. But Macbeth only knows that one of the murderers told him Banquo was dead.
Ay, my good lord. Safe in a ditch he bides,
With twenty trenched gashes on his head,
The least a death to nature.
Macbeth fears Banquo. When he sees him occupying his place at the banquet table he may think, at least momentarily, that Banquo has organized an elaborate plot against him--that the murderers were working for Banquo from the beginning, or else that Banquo had managed to "turn" them and make them double agents.
When Macbeth asks, "Which of you have done this?", he addresses the lords and guests at the feast. Lennox has just tried to show Macbeth to his seat, only Macbeth cannot see that a seat is empty. Instead, he sees the ghosty visage of Banquo. When he accusingly asks the Lords which one of them did this, Macbeth, in all his paranoia, supposes that one of the Lords arranged a deathly looking Banquo to be present at the table--as if to make an accusation of Macbeth's guilt. Shakespeare's play does not include details as to what sort of set-up Macbeth imagines the Lords to be capable of; rather the vagueness of the set directions allows the audience to use their imagination.
Of course, the Lords have no idea what Macbeth is talking about! They reply, "What, my lord?" (61) No one can see Banquo's ghostly image, save Macbeth whose guilty conscience convicts him at once. Even still, Macbeth reacts in denial, hastily ordering Banquo's ghost "never to shake thy gory locks" at him (62-62). The imagery of the gory locks suggests that Banquo's ghost must be disgustingly gory.