Final answer:
Lady Macbeth covers for Macbeth's erratic behavior during the banquet by telling the guests that he has suffered from these sudden episodes since youth, suggesting it's a harmless condition and asking them to ignore his outbursts for his well-being.
Step-by-step explanation:
Lady Macbeth explains Macbeth's strange behavior at the banquet by suggesting he has a momentary ailment, one she claims is common and has been with him since his youth. She attempts to dismiss his panicked vision of Banquo's ghost as a mere episode, addressing the guests with the lines:
‘Sit, worthy friends. My lord is often thus,
And hath been from his youth. Pray you, keep seat.
The fit is momentary; upon a thought
He will again be well. If much you note him,
You shall offend him and extend his passion.
Feed, and regard him not.’
This explanation is one of her attempts to retain control over the quickly deteriorating situation, as seen previously where she has to act decisively to cover for Macbeth's errors or lack of resolve. For example, when Macbeth forgets to leave the daggers with the grooms, Lady Macbeth chastises him and takes charge of smearing the grooms with blood:
‘Infirm of purpose!
Give me the daggers. The sleeping and the dead
Are but as pictures. ’
These lines underscore her attempts to manage the perception of Macbeth's actions to others and her role in steering the consequences of their joint venture.