Lady Macbeth chides her husband on lacking the courage to do the deed, and because Macbeth is a warrior above everything everything else, her comments manipulate him a good deal. Indeed, "manliness" is an idea the play interrogates in various ways, such as when Macbeth says "I dare do all that may become a man/Who dares do more is none" (1.7.46-47). This is the notion that his wife use against him: "When you durst do it, then you were a man / And to be more than what you were, you would/Be so much more a man" (1.7.51-53). Some productions show Lady Macbeth kissing her husband, seducing him sexually even while she questions and then builds up his manhood to convince him to kill Duncan. In short, it is by means of his manhood, fundamental to his notion of himself as warrior, that Lady Macbeth manipulates her husband.
Because the witches offer Macbeth the hope of getting something that he didn't think was within his reach, they are often considered the force that manipulates Macbeth into killing Duncan, but I would agree with malibrarian in that I don't think they make Macbeth do anything that he isn't already disposed towards doing.
I would also agree that the real manipulation comes from the lady, but I would cite a different part of the play as evidence--Macbeth has already decided that he has no good reason to kill Duncan, and even informs Lady Macbeth of his decision (he says Duncan has been good to him, he has sworn an oath of loyalty, Duncan is a guest in his house, and, finally, everyone loves Duncan. He decides that he may get what he wants after all, because he is fated to do so, not because he has killed to get it. Lady Macbeth's response is one of the most manipulative speeches in all of literature: she asks him if the hope that he 'dressed himself' in the night before was "drunk"--and then she goes on to question his love for her (from this time on shall I account thy love) and his courage. He's had enough, but she is relentless--she questions his manhood, saying he would be a man when he's done the deed, not before. Finally, she questions his faithfulness to her. Her most famous move is when she says she'd rather rip her nursing baby from her nipple and smash its brains out than break a promise to him. So subtle. But effective, because soon, he acts.
The witches seem to be the best manipulators around. They take a man, Macbeth, who was being rewarded by King Duncan for his loyalty and courage in defending the Scottish realm, and manage to turn him into a murdering, power-hungry evil being, simply by saying, "someday you'll be king." (One could say, though, that Macbeth had to have some ambition in him already if he could be so easily swayed, while his friend, Banquo, stayed loyal to Duncan, despite being told his descendents would someday be kings.)
Lady Macbeth is, in my opinion, an even better example of a manipulator. She is determined that her husband not become weak in fulfilling their plan to kill Duncan and take the throne for themselves. She worries that Macbeth is "too full o' the milk of human kindness" - that he is really too good inside to follow through with killing Duncan. (Ironic, then, that she is the one later who cannot make herself stab Duncan as he reminds her too much of her father.)
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