Act 1, scene 5 of Shakespeare's Macbeth is Lady Macbeth's first appearance in the play, and it's notable that she first appears entirely alone. All of the major characters in Macbeth—King Duncan, Macbeth, Banquo, Malcolm, Macduff—first appear with other characters. Few major characters enter this late in Shakespeare's plays—an exception, Macduff, doesn't enter Macbeth until act 2, scene 1—and even fewer characters make their first entrance alone. Richard, Duke of Gloucester—not-so-coincidentally another villain—enters alone in act 1, scene 1 of Richard III.
Lady Macbeth enters the scene reading a letter from Macbeth about his meeting with the three witches in which they prophesize that he "shalt be King hereafter!" After a brief interruption by a messenger with information that King Duncan will soon arrive at her castle, Lady Macbeth gives her most striking speech in the play. She calls on evil spirits to remove her humanity from her so that she'll be able to murder Duncan without any pangs of conscience or any sense of regret or remorse.
Shakespeare usually has highborn characters communicate with one another in verse, rather than in prose, which Shakespeare assigns to lower-born characters. However, Macbeth, a Scottish noble, writes his letter to Lady Macbeth in prose.
This give a sense of Macbeth's role in the play up to this point—that of a soldier, albeit a general, in Duncan's army and therefore subservient to Duncan—and it shows a certain sense of urgency on Macbeth's part in conveying the information about the witches to Lady Macbeth as clearly and directly as possible. Macbeth writes in haste, and he's already on his way back to Lady Macbeth at his castle. He simply doesn't have time to convey his thoughts in proper, poetic, courtly form.
Lady Macbeth uses repetition to emphasize particular points in her speeches. Referring to Macbeth, she considers the possibility of his reluctance to kill Duncan.
LADY MACBETH. What thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win.
(act 1, scene 5, lines 17–19)
For extra emphasis, Lady Macbeth contrasts "highly" with "holily," uses the multiple alliteration within those two words, and also concludes the line with three-word alliteration, "wouldst wrongly win."
Lady Macbeth implores the evil spirits to "make thick [her] blood," and she calls on elements in nature, and the imagery they bring to mind, to hide her actions: "Come, thick night."
She instructs Macbeth "to beguile the time, / Look like the time," adding the assonance in "beguile" and "like," the alliteration of "Look like," and a simile, "Look like the time," for good measure.
LADY MACBETH. [L]ook like the innocent flower,
But be the serpent under't.
(act 1, scene 5, lines 70–71)
In this famous admonition to Macbeth, Shakespeare—through Lady Macbeth—employs similes, metaphors, alliteration, imagery, symbolism, diction, and irony, all in just two lines.
The first part of Lady Macbeth's instructions to Macbeth— "Look like the innocent flower"—is a simile, as well as a metaphor of Macbeth as an "innocent flower." She repeats the alliteration—"Look like"—that she used just two lines previous and adds the imagery of the flower. Lady Macbeth wants Macbeth to appear as innocent and harmless as a flower to Duncan and those around him so that no one suspects his intentions towards Duncan and the throne.
The second half of Lady Macbeth's admonition to Macbeth is also a simile, a metaphor of Macbeth as a "serpent," contains alliteration—"But be"—and adds the imagery of the serpent. Shakespeare's diction, his word choice of "serpent" rather than "snake," emphasizes Macbeth's evil intentions.
With his use of "serpent," Shakespeare compares the symbolism of the common negative perception of a serpent with the pure evil and treachery of the serpent in the biblical story of Adam and Eve in the Garden of Eden.
The irony of these lines is that in instructing her husband to be a "snake in the grass" with regard to Duncan and his aspiration to the throne of Scotland, it is actually Lady Macbeth who takes the role of the serpent in the Garden of Eden, who tempts Macbeth not with great knowledge but with great power.