The plan is not necessarily all Lady Macbeth's. She and her husband have discussed killing King Duncan in the past. This is revealed in her response when her husband tells he has decided to give up the idea altogether.
What beast was 't then,
That made you break this enterprise to me?
When you durst do it, then you were a man;
And to be more than what you were, you would
Be so much more the man. Nor time nor place
Did then adhere, and yet you would make both.
They have made themselves, and that their fitness now
Does unmake you.
So it was Macbeth himself who originally thought of murdering Duncan. What he probably had in mind was inviting the King to visit him at Elsinore and then murder him in his sleep. It would have been hard to pull off such an assassination any other way. Then the fighting occurring in the vicinity of Macbeth's stronghold forced Duncan to request accommodation there. Macbeth and his wife must have discussed how they would proceed if they could somehow get the King to sleep under their roof. The joint plan was to knock out the grooms, murder Duncan, and then pretend to have been sound asleep when the body was discovered. They had to make the grooms look guilty by using the men's daggers for the crime and then smearing their faces with blood. Lady Macbeth seems to have taken the whole idea more seriously than her husband, who was only speculating. She is right in saying that the time and place "have made themselves, and that their fitness now does unmake (i.e., unnerve) you." He was thinking in terms of years and finding just the right opportunity to invite the King to enjoy their hospitality. His wife is obviously more practical, direct, decisive, and also more ambitious. However, her husband is the one who has to do the actual killing. That's his business. He's very good at it. But it explains why he is more hesitant about acting. He's used to killing men in battle, not sneaking into their bedrooms and murdering them in their sleep.
Lady Macbeth is the ultimate expression of a woman’s desire to hold power. She is the powerhouse in the relationship which influences her husband to begin his downward spiral into the insane world of Macbeth. Without the influence and quick thinking of Lady Macbeth there would have never been a murder.
Lady Macbeth clearly lays out a plan for her husband that serves to cover all bases. Her plan ensures that any thwarters are framed and the murderous couple appears innocent and free from blame. She states :
“We fail! But screw your courage to the sticking-place, And we'll not fail. When Duncan is asleep-- Whereto the rather shall his day's hard journey Soundly invite him--his two chamberlains Will I with wine and wassail so convince That memory, the warder of the brain, Shall be a fume, and the receipt of reason A limbeck only: when in swinish sleep Their drenched natures lie as in a death, What cannot you and I perform upon The unguarded Duncan? what not put upon His spongy officers, who shall bear the guilt Of our great quell?”-Shakespeare, Macbeth Act 1, scvii
Her plan outlines a fearless attack on Duncan while he sleeps in their home. After she points out her husband’s lack of courage, she begins to describe how she will provide the “chamberlains” or the king’s official bodyguards with enough alcohol (wine and wassail) to cloud their memory and put them to sleep. Once ensured of their drunken sleep, Duncan would be left unguarded allowing Macbeth and Lady Macbeth the opportunity the needed to murder him. Lady Macbeth plans that they would perform the deed using the bodyguards own daggers. She explains to her husband that the bloody daggers would be left in the bodyguard’s hands to ensure the drunken bodyguards would take the blame for the crime.
The ultimate plan of betrayal and deceit! While the bodyguards are framed, Lady Macbeth plans that she and Macbeth will be “mourning” the death of Duncan.
It is a rather stunning introduction to the wife of Macbeth that the audience receives in Act I, Scene 5. Macbeth's "dearest partner of greatness," a foil to her husband,--even considered a doppelganger to Macbeth--Lady Macbeth reads a confidential letter regarding his recent experiences in battle and his encounter with the witches who have "more in them than mortal knowledge" (1.5.3). Her immediate reaction is that Macbeth is "too full o' th'milk of human kindness" to act in most expedient manner even though he would approve of the result; therefore, Lady Macbeth desires that he make haste in returning so that she can persuade him and allay his fears or contradict any argument that he may propose against seizing the crown.
When she unexpectedly learns that King Duncan is coming to the Macbeth's castle, Inverness, Lady Macbeth seizes the moment in her soliloquy:
The raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. (1.5.28-30)
Quickly, she summons the supernatural herself, calling upon the spirits to unsex her and fill her full of "direst cruelty" (1.5.33). In other words, Lady Macbeth wishes to become less of a woman and more like a man since violence is associated with men as warriors. Also, she asks the deadly spirits to remove from her any womanly feelings of pity and compassion, exchanging her "milk" of kindness for the "gall" of bitterness and cruelty so that she will have no misgivings about her deadly and savage intentions. Further, she beckons night to fall in order to cloak the knife in darkness so that no one will be alerted to the murderous act and cry out:
....Come, thick night,And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,Nor heaven peep through the blanket of the darkTo cry “Hold, hold!” (1.5.40-44)
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (1.7.72-74)