Lady Macbeth defines the concept of manhood and masculinity by being bold, hostile, and violent. When Lady Macbeth initially receives her husband's letter regarding the witches' presumably favorable prophecies, she calls upon evil spirits to "unsex" her and take her "milk for gall." In her soliloquy, Lady Macbeth reveals her perspective on the female gender and feels that she must shed her gentle nature in order to be callous and cruel like a man. In act 1, scene 7, Macbeth tells his wife that he has reservations about assassinating King Duncan, and Lady Macbeth once again comments on gender roles by telling her husband,

When you durst do it, then you were a man; And to be more than what you were, you would Be so much more the man. (Shakespeare, 1.7.50–51)

Lady Macbeth's comments support her belief that manhood implies acting resolute, violent, and bold.

In act 4, scene 3, Macduff receives the terrible news that his entire family has been slaughtered by Macbeth's assassins. Malcolm encourages Macduff to cure his grief by seeking revenge. Malcolm also tells Macduff, "Dispute it like a man" (Shakespeare, 4.3.226). Malcolm's response to Macduff's misfortune indicates that he also believes manhood implies that one should act aggressive, fearless, and hostile. He is younger than Macduff and possesses a naive perspective of masculinity and manhood. Macduff reveals his opposite view by telling Malcolm,

I shall do so, But I must also feel it as a man. (Shakespeare, 4.3.227)

Unlike Lady Macbeth and Malcolm, Macduff recognizes that the concept of manhood includes the ability to sympathize, reflect, and mourn. Macduff's concept of manhood is significantly more flexible and reflects his belief that men should be in touch with their feelings and have the ability to exercise gentleness. Overall, Lady Macbeth and Malcolm's concept of manhood is rigid and includes one's ability to act resolute, cruel, and violent, while Macduff's concept is more well-rounded, flexible, and sound.

For Macbeth, a man should be composed of bravery and boldness. His wife seems to agree, as she tries to talk him back into murdering the king, Duncan, his relative and friend, using a number of pretty harsh arguments. She does actually say "When you durst do it, then you were a man"—essentially, she assumes that a sense of daring is what makes the man. In response to her arguments, he tells her,

Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (1.7.83-85)

In other words, Macbeth tells Lady Macbeth that she should only give birth to boys because her nature is much more suited to males than females. What he calls her "undaunted mettle" refers to her temperament: she is resolute in the face of danger, and she will not be persuaded from her purpose. This, for him, is what men should be.

For Lady Macbeth, also, a man should maintain the utmost self-control while still remaining courageous and daring. When he sees Banquo's ghost at their dinner party and begins to quake and cower in front of his guests, she asks him, 

Are you a man?
. . . 
O, proper stuff! 
This is the very painting of your fear.
This is the air-drawn dagger which you said
Led you to Duncan. O, these flaws and starts,
Impostors to true fear, would well become
A woman's story at a winter's fire,
Authorized by her grandam. Shame itself!  (3.4.70-79)

In asking if he is a man, she implies that he isn't acting like one. She thinks what he's saying about the ghost is total nonsense and just another hallucination because he's so cowardly and fearful. She says that his behavior is that of a woman who is telling a scary story, sitting by the fire with her grandma. She says that he ought to feel ashamed of himself.  

Malcolm defines the kind of man he is, his character, in this excerpt from his conversation with Macduff in Act IV, Scene iii:

[I] scarcely have coveted what was mine own,

At no time broke my faith, would not betray

The devil to his fellow, and delight

No less in truth than life.

Malcolm defines manhood in terms of personal integrity.

For Macduff, however, manhood is a matter of strength and responsibility. When he learns his family has been slaughtered, he is overcome by grief. Malcolm tells him to "[d]ispute it like a man." Macduff says he will be strong, but he cannot ignore his feelings: "I must also feel it as a man." Macduff's family was murdered as a result of his opposition to Macbeth, and he takes responsibility for their deaths:

Sinful Macduff,

They were all struck for thee! Naught that I am,

Not for their own demerits but for mine

Fell slaughter on their souls. Heaven rest them now!

Unlike Macduff and Malcolm, Lady Macbeth does not define manhood in terms of very positive traits of character. For her, being a man means being cruel, this idea made plain in her desire to gain masculine traits to replace her own feminine nature:

Come, you spirits

That tend on mortal thoughts, unsex me here,

And fill me, from the crown to the toe, top-full

Of direst cruelty! Make thick my blood,

Stop up th' access and passage to remorse

That no compunctious visitings of nature [pity]

Shake my fell [savage] purpose . . . .

Later, she fears that Macbeth will not be cruel enough to follow through with Duncan's murder, and she shows contempt for Macbeth, "so green and pale," when he expresses fear for what they are planning to do.