How does William Shakespeare, in Macbeth, use foresight to instill fear in Macbeth and/or Lady Macbeth so that he/she makes life-altering decisions?
This is an interesting question. There are several incidents where fear gleaned from foresight instills fear in the husband and/or wife.
In Shakespeare's Macbeth, the source of foresight comes from the supernatural. Macbeth twice receives predictions from the witches that create fear within him, and his subsequent actions are life-altering.
When the witches give Macbeth and Banquo their predictions, Macbeth decides (with Lady Macbeth) to kill the King in the hope that somehow the throne will come to him. However, once this is done, Macbeth becomes obsessed with keeping the crown. One concern he has comes from Banquo's knowledge of the witches' words to him. The other is from Banquo's predictions that state that while Banquo will not be king, he will father a line of kings. This prediction causes Macbeth fear. He will kill Banquo to keep him quiet, but his fear that Fleance will carry on Banquo's line to father kings makes Macbeth plan to kill Fleance.
…There's blood upon thy face.
’Tis Banquo's then.
’Tis better thee without than he within.
Is he dispatch'd?
My lord, his throat is cut; that I did for him... (III.iv.13-17)
Most royal sir,
Fleance is 'scaped. (21-22)
Whereby there is little to decisively implicate Macbeth after Duncan's death, Banquo's death and the attempt on Fleance's life raise concerns. This act, driven by Macbeth's fear, is life-altering, for after this his lords begin to note that those closest to Macbeth end up dead, and this places a whisper of suspicion in the minds of those around him. Lennox recognizes the deaths of those around Macbeth, and with some sarcasm, notes that it is dangerous to walk out late at night as Banquo did—the inference is clear that Macbeth has lost all credibility and rumors carry news of this one-time hero, who has turned into a tyrant and traitor:
Things have been strangely borne. The gracious Duncan
Was pitied of Macbeth: marry, he was dead.
And the right valiant Banquo walk'd too late,
Whom, you may say, if't please you, Fleance kill'd,
For Fleance fled. Men must not walk too late. (III.vi.3-7)
At the banquet, Macbeth tells his wife that Macduff did not come to the banquet as summoned. He notes that he will check with the spy he pays in that nobleman's house, and that he will also return to the witches. When he meets them, the first warning he receives is about Macduff:
Macbeth! Macbeth! Macbeth! Beware
Beware the Thane of Fife. (IV.i.78-80)
The second apparition tells Macbeth that no one born of a woman can harm him, and he says that he has no reason to fear Macduff. However, he is not completely sure, and—once again fearful of a threat to his place as King—he decides (just in case) to send murderers to kill Macduff and his entire family.
Macduff is not there. He is visiting the exiled Malcolm to see if he will return to Scotland to rid the land of Macbeth. Upon learning of his family's destruction, Macduff is ever more committed to not only overthrow Macbeth, but also be the one to kill him. Macbeth's action is life-altering, for Macduff is the man who is not "of woman born," and therefore is the one who can—and will—kill him.
Despair thy charm…
Macduff was from his mother's womb
Untimely ripp'd. (V.viii.17, 19-20)
Macbeth allows his fear affect his decisions, which lead to his destruction.