First, both of them exhibit evil characteristics; this is revealed in the plot to murder Duncan and then later when Macbeth has his best friend murdered and Lady Macduff and her children murdered. And let's not forget Lady Macbeth's famous soliloquy in Act I, scene v, where she asks the "spirits that tend on mortal thoughts" to "unsex" her so she can murder without feeling any compunction. And don't forget how evil Lady Macbeth reveals herself to be when in Act I, scene vii, Macbeth tells her that they "will proceed no further in this business"(I. vii), the "business" being the murdering of King Duncan, She convinces him to do it by saying
I have given suck, and know
How tender 'tis to love the babe that milks me;
I would, while it was smiling in my face,
Have plucked my nipple from his boneless gums,
And dashed the brains out, had I sworn as you
Have done to this.
Macbeth quickly changes his mind, which shows just how easily he can be persuaded by his wife, and just how weak-minded he is. But the evilness of Lady Macbeth is profoundly revealed here; she would kill her own baby before she would back out of killing Duncan had she promised to do so.
Secondly, they both are deceptive; this is shown in their ability "to beguile the time." They both "beguile" King Duncan prior to the murder, and they are able to fool most of the people after Duncan's death with their niffy display of emotion and Lady Macbeth's fainting scene. And don't forget how deceptive Lady Macbeth is in Act III, when Macbeth is in a state of hysteria when he sees Banquo's ghost at dinner; there she tells the guests not to question Macbeth for questions will only enrage his fit, which she says he has been suffering with since his childhood.
Thirdly, their nefarious ways are fueled by their ambition to become king and queen; it is their ambition that leads them to be so deceptively evil. Macbeth can't stop thinking about becoming king after the witches' prophecy comes true about Macbeth becoming the Thane of Cawdor, for along with that prophecy, they tell him that he will be king too. From that moment on, Macbeth is driven by ambition. It appears that Macbeth has always been driven by ambition; look at the way he fights in battle; he splits Macdonwald in half and then chops off his head. The way he fights and the fact that he won the battles all suggest that Macbeth is highly ambitious.
Lady Macbeth is not without ambition, for after reading Macbeth's letter which gives her an account of the fateful meeting with the witches, she states that Macbeth "shalt be / What thou art promised." If you listen to the tone in these words, there is a proclamation being made here by her; the proclamation is that Macbeth will become king if it's the last thing she does, which, by the way, is. And to further amplify her ambition, she attempts to sell her soul to the "spirits that tend on moral thoughts," and she does so in a most perverted manner for she beckons them to "come to [her] woman's breasts, And take [her] milk for gall." How's that for ambition?
I would say Macbeth has two main characteristics. First, he is very ambitious -- he wants power. You can see this in the fact that he believes the witches' prophecy and then goes and acts on it (kills Duncan). Second, he is not very strong-willed. You can see this in how much the idea of killing Duncan scares him before hand (the dagger vision) and how guilty he feels after (can't say "amen").
Lady Macbeth is more forceful, especially early in the play (she pushes Macbeth to kill Duncan). But then later on she starts to feel guilty for what she has done as well ("out, out, damned spot").