Soon after the witches hail him as "Thane of Glamis," "Thane of Cawdor," "and king hereafter!" (1.3.50), Macbeth receives the news that he has been named Thane of Cawdor. This news throws him into a reverie, in which he says to himself, "Two truths are told, / As happy prologues to the swelling act / Of the imperial theme" (1.3.127-129). Macbeth's metaphor is dramatic, or musical; he seems to be imagining himself as making a grand entrance as king, or maybe as an emperor, a king of kings.
Just as the King is commenting on the treachery of the former Thane of Cawdor, in comes the new Thane of Cawdor, Macbeth. The King greets Macbeth as "worthiest cousin!" (1.4.14) and says in several different ways that he can't thank him enough. Macbeth answers with heroic modesty that "The service and the loyalty I owe, / In doing it, pays itself" (1.4.22-23). That is, it's payment enough to know that he did the right thing as a loyal servant of the King. Then Macbeth adds, Your highness' part
Is to receive our duties; and our duties
Are to your throne and state [stateliness, dignity] children and servants, Which do but what they should, by doing every thing
Safe toward your love and honour. (1.4.23-27)
"Safe toward" means "to secure" or "to safeguard"; the idea is that it is every subject's duty to do everything he can for the king, both to keep the king safe and to earn the king's love and respect. Macbeth's speech pictures King Duncan as the loving father of a happy family, but Macbeth is already thinking about killing him.
When Lady Macbeth receives her husband's letter about the witches' prophecies, she is only worried that her husband is "too full o' the milk of human kindness / To catch the nearest way" (1.5.17-18). But she's sure she has no such problem, and she's eager for the chance to make him see things her way. Holding the letter, and speaking to Macbeth (even though he hasn't arrived yet) she says, "Hie thee hither, / That I may pour my spirits in thine ear; / And chastise with the valour of my tongue / All that impedes thee from the golden round," (1.5.25-28). We might say that she's going to nag him, but she believes that she is going to enable him to reach his potential. She will "chastise" (make him ashamed of) everything in him that prevents him from being evil enough to be king. Shortly, Macbeth appears and tells her that King Duncan will be staying with them that night. Lady Macbeth declares that King Duncan will never leave their castle alive and advises Macbeth to be a good hypocrite. He must give the king a warm welcome, the better to kill him that night. Apparently Macbeth shows a little reluctance, because she says, He that's coming
Must be provided for: and you shall put
This night's great business into my dispatch;
Which shall to all our nights and days to come
Give solely sovereign sway and masterdom (1.5.66-70)
"Sovereign" means not only "kingly" but also "absolute." Lady Macbeth is telling her husband that if he will only do as she tells him, they will be king and queen, with power over all. To her, the essence of kingship is ruthless power.
When King Duncan is greeted by Lady Macbeth, he makes a little joke about the social difficulties of being king. He says to her:
See, see, our honour'd hostess!
The love that follows us sometime is our trouble,
Which still we thank as love. Herein I teach you
How you shall bid God 'ield us for your pains,
And thank us for your trouble....