Macbeth is about how the quest for power and, in Macbeth's own case, how his "vaulting ambition" (Act I, scene vii, line 26) causes him, a valiant hero who has loyally defended his king, to do terrible things.
Macbeth is praised by the King (Duncan) and is granted the title of Thane of Cawdor but, because the witches have predicted that not only will Macbeth be Thane but he will also eventually be king, Macbeth wonders whether he will need to do anything to ensure that the prediction comes true. This is the first example of how he changes in Act I. It seems that the witches have such an effect on him that he begins to think about getting rid of anyone in his way, not only Duncan, but Duncan's heir (Malcolm), too. He says, "...For in my way it lies...Let not light see my black and deep desires" (I.iv.50). He tells the audience that he has these "black" thoughts which he knows are wicked. He continues to change from the valiant soldier and, although he will be affected by guilt after killing Duncan (and forgetting to leave the daggers behind) and will be remorseful as he wishes to "wake Duncan," by the time he has had Banquo killed, he no longer recognizes the guilt and is driven by his need to be king at all costs. He suggests that Lady Macbeth, in Act III, scene ii, line 46, will "applaud the deed" when she hears that Banquo is dead. He does not expect to make any mistakes this time (like he did by forgetting to leave the daggers behind when he killed Duncan). This reflects a radical change in Macbeth who no longer relies on Lady Macbeth as much as he did when he killed Duncan, whom he killed when she told him that he was less than a man otherwise; (see Act I, scene vii, lines 49-50). She does, however, still protect him when Banquo's ghost appears. We see Macbeth completely changed when Lady Macbeth dies and Macbeth realizes that the witches are "juggling fiends" (V.viii.19) who have not protected his interests at all. He will not surrender, thinking it is more noble to fight to the end even though he cannot win.
Lady Macbeth is another character that completely changes. She is scheming from the beginning, recognizing that Macbeth is not entirely wicked and that she will have to "chastise with the valor of my tongue All that impedes thee..." (I.v.24-25), meaning that she knows that she will have to mock him and plan everything if they are to succeed; she will have to convince him to carry out the plan by removing any obstacles. She worries about Macbeth but begins to change when she realizes that he is not involving her in his schemes and, in fact, is perhaps going too far. She encourages Macbeth to stop, telling him "You must leave this" (III.ii.55). Her descent into madness is confirmed when she babbles about the "damned spot" (V.i.32) and, having staunchly protected Macbeth, says she can no longer defend him. Ultimately, the doctor says, "more needs she the divine..." (line 72) when there is no hope for her.
The witches also transform from simple mischievousness on a day on which "fair is foul" (I.i.10) into manipulative "fiends" as they realize that they can encourage Macbeth's evil. From having only told Macbeth that he will become king without going into too much detail, they tell him things that will encourage his foolishness. They recognize that "something wicked this way comes" (IV.i.45) and intend to have some fun with Macbeth by feeding his obsession by telling him "none of woman born..." (IV.i.80).