When we first hear about Macbeth from the sergeant's report, we are led to believe that he is very much a person who does only what he believes is right. Furthermore, when he first appears in the play, his fellow nobleman, Banquo, accompanies him. Given this, we would think that he does what is right, and makes all his own decisions. However this belief is proven wrong. Although Macbeth starts off as a loyal subject of Duncan, he is ambitious and this is a weakness, which allows him to be manipulated by a few factors in the play.
From the beginning of act 1 scene 5 till the murder of Duncan in act 2 scene 2, it is evident that Lady Macbeth manipulates and convinces Macbeth into murdering Duncan. This is basically the only part of the play where Lady Macbeth has control over Macbeth. After Duncan's murder, Macbeth is partly driven by evil powers.
Proof of Lady Macbeth understanding Macbeth's thoughts and feelings are visible in her thoughts "Thou wouldst be great, Art not without ambition, but without the illness should attend it." (1.5.19-21). From the very same speech and small implications from the letter, we can tell that Macbeth was ambitious enough to want to be the King, but would not think of murdering Duncan. Because Lady Macbeth knew this, she also knew that she would have to push Macbeth into performing the deed and she starts by telling him "Thy letters have transported me beyond this ignorant present and I feel now the future in the present" (1.5.57-59) the moment they meet. From Macbeth's reply "We will speak further" (1.5.69) he is obviously noncommittal, but was already thinking about it. The first step of manipulation has started.
Not much further in the play, we see that Macbeth...