What techniques does Lady Macbeth use to convince her husband to kill Duncan in Macbeth?
When Macbeth states that "We will proceed no further in this business," Lady Macbeth uses rhetorical questions to persuade her husband. She questions his ambition and asks whether Macbeth was drunk when he expressed the desire to assassinate Duncan. She questions his courage, asking if that which he had previously so bravely declared has now been overtaken by a lack of courage and conviction. She uses the words "green" and "pale," enquiring whether the idea of murder has now made him sick and cowardly.
She then states that she would from then on measure Macbeth's love for her in terms of how he acts. She continues asking rhetorical questions and wants to know if her husband is now afraid of putting his words into action. She questions his ambition to obtain "the ornament of life" (the crown) - would he rather live a coward the rest of his life than obtain that which he so much desires? She uses a reference to the "poor cat in the adage" referring to the fact that a cat likes fish, but does not want to wet her paws. Is Macbeth prepared to take risks to achieve his ambition?
When Macbeth states that he is not a coward and would do anything to prove his masculinity, Lady Macbeth then asks him what beast had made him break the promise that he had made her. When he made the promise he was more of a man than he currently is. She uses another comparison, this one a vile example of her determination. She states that if she had promised to murder her suckling babe she would do so and rip her own breastfeeding child from her breast and dash out its brains. This horrendous image indicates how determined Lady Macbeth is to go through with Duncan's murder. She would be remorseless. Obviously, she is using a very persuasive technique and wishes her husband to feel embarrassed and cowardly about not wishing to go through with the planned assassination.
When Macbeth expresses the concern that they could fail, Lady Macbeth tells him to "screw your courage to the sticking place," suggesting that her husband pluck up enough courage to continue with the deed. To encourage him even more, she suggests that she would get Duncan's guards drunk so that not only would they be able to kill Duncan without fear of being discovered or disturbed, but also that they would implicate the guards in their king's murder. They will then weep and wail to make their grief about Duncan's death known to all - a further indication that they were not involved but are shocked by the travesty.
Macbeth is now convinced and expresses his determination to commit the deed.
The techniques that Lady Macbeth has used to manipulate her husband have been well identified and explained in the above answers. I believe the question remains of why Shakespeare chose to have her use all her persuasive powers to make Macbeth go through with a murder he did not want to commit, a murder he deplores even while he is committing it. He tells her:
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (1.7)
Just prior to that, in a soliloquy, he has gone over all the reasons why he should not kill Duncan. It is obviously against all his instincts and inclinations to do what his wife wants. It would seem that Shakespeare was trying his best to make Macbeth a somewhat sympathetic figure by casting the blame for the murder as much as possible on his wife. We feel that Macbeth would never have gone through with the bloody deed if his wife hadn't talked him into it. She uses "sexual blackmail" when she says:
From this time
Such I account your love.
She is suggesting that she will withhold her sexual favors if he doesn't do as she wishes. She will be be cold, indifferent, uncommunicative, moody, unapproachable. Women have been known to do such things to get their way.
So if Shakespeare succeeds in making us feel a little bit sympathetic for the poor man, we also despise him commensurately for being such a wimp. He is a good example of an uxorious husband, one who is dominated by his wife. By the time he faces Macduff on the battlefield in the last act, we have had enough of him. He does not have just one Aristotelian fault but a dozen. He is a murderer, a tyrant, an incompetent ruler, a bully, a psychopath, and a henpecked husband. The only thing he has in his favor is his courage. He is actually determined to fight with Fate itself, personified as an enemy warrior.
Rather than so, come, Fate, into the list,
And champion me to the utterance! (3.1)
In fact, the main conflict in the play might be described as "man against fate."
In Macbeth, Lady Macbeth chides Macbeth's manhood in order to persuade him to kill King Duncan. When Lady Macbeth receives the letter from her husband explaining the witches' prediction, she immediately realizes that Macbeth is too "full of the milk of human kindness" to go after his ambitions. At this point, Lady Macbeth decides to become a motivating factor in helping Macbeth gain the throne. She sets up a plan and presents it to Macbeth. He, however, finds Duncan a good man and a good kings, so he wavers in his decision to carry out the murder. Lady Macbeth steps in and tells Macbeth that he would be more of a man if he would just go after his ambitions and carry out whatever actions are necessary to make his goals reality. She tells him that he would be a coward to let this opportunity pass him. With this, Macbeth is persuaded to act.