Because we first hear of Macbeth in the wounded captain's account of his battlefield valor, our initial impression is of a brave and capable warrior. This perspective is complicated, however, once we see Macbeth interact with the three witches. We realize that his physical courage is joined by a consuming ambition and a tendency to self-doubt--the prediction that he will be king brings him joy, but it also creates inner turmoil. These three attributes--bravery, ambition, and self-doubt--struggle for mastery of Macbeth throughout the play. This complexity adds another dimension to the character of Macbeth, making him much more interesting.
Before he kills Duncan, Macbeth is plagued by worry and almost aborts the crime. It takes Lady Macbeth's steely sense of purpose to push him into the deed. After the murder, however, her powerful personality begins to disintegrate, leaving Macbeth increasingly alone. He fluctuates between fits of fevered action, in which he plots a series of murders to secure his throne, and moments of terrible guilt and absolute pessimism. These fluctuations reflect the tragic tension within Macbeth: he is at once too ambitious to allow his conscience to stop him from murdering his way to the top and too conscientious to be happy with himself as a murderer. This constant inner struggle provides a deep insight into the Macbeth, making the reader sympathise with him while strongly disapproving of his actions. This ability to get the reader involved makes Macbeth much more remarkable than other characters in the play.
As things fall apart for him at the end of the play, he seems almost relieved--with the English army at his gates, he can finally return to life as a warrior, and he displays a kind of reckless bravado as his enemies surround him and drag him down. In part, this stems from his fatal confidence in the witches' prophecies, but it also seems to derive from the fact that he has returned to the arena where he has been most successful and where his internal turmoil need not affect him--namely, the battlefield. Unlike many of Shakespeare's other tragic heroes, Macbeth never seems to contemplate suicide: "Why should I play the Roman fool," he asks, "and die ... On mine own sword. Instead, he goes down fighting, bringing the play full circle: it begins with Macbeth winning on the battlefield and ends with him dying in combat. Simply because Macbeth has been so important in the play, the reader has been able to empathise with Macbeth many times, and feels a sense of familiarity with him- every time he appears in the play, the reader is curious to know the state of mind that he is currently in. This is in great contrast to other characters in the play. For example Banquo's son- only appearing two or three times during the play, we are only able to gather that he is in fact Banquo's son, and that he is swift in escaping from the three murderers. His character has not been developed at all, and is simply a tool of the play that is necessary for the storyline. Banquo's sons, along with the majority of the other characters in the play have similar shortcomings, and are of little interest to the audience/reader. The lack of inner conflict also makes the other characters very boring.
Lady Macbeth is one of Shakespeare's most famous and frightening female characters. When we first see her, she is already plotting Duncan's murder, and she is stronger, more ruthless, and more ambitious than her husband. She seems fully aware of this and knows that she will have to push Macbeth into committing murder. At one point, she wishes that she were not a woman so that she could do it herself. This theme of the relationship between gender and power is key to Lady Macbeth's character: her husband implies that she is a masculine soul inhabiting a female body, which seems to link masculinity to ambition and violence.
Lady Macbeth manipulates her husband with remarkable effectiveness, overriding all his objections; when he hesitates to murder; she repeatedly questions his manhood until he feels that he must commit murder to prove himself. Lady Macbeth's remarkable strength of will persists through the murder of the king--it is she who steadies her husband's nerves immediately after the crime has been perpetrated. Afterward, however, she begins a slow slide into madness--just as ambition affects her more strongly than Macbeth before the crime, so does guilt plague her more strongly afterward. By the close of the play, she has been reduced to sleepwalking through the castle, desperately trying to wash away an invisible bloodstain. Once the sense of guilt comes home to roost, Lady Macbeth's sensitivity becomes a weakness, and she is unable to cope. Significantly, she apparently kills herself, signaling her total inability to deal with the legacy of their crimes. Like Macbeth, lady Macbeth's behavior and feelings alter throughout the play, making for very interesting viewing. We also get an in depth view into Lady Macbeth's personality through close focus on her emotional struggle.
Hence, it appears that the villains in Macbeth are the only characters that are interesting to the viewer. The comprehensive development of Macbeth and Lady Macbeth throughout the play, and the inner conflicts between good and evil within these two characters render them much more interesting than the other characters in Macbeth. The lack of complexity and depth in the other characters make them uninteresting to the viewer, resulting in Macbeth and Lady Macbeth being the only two interesting characters in Macbeth.