It's true that the dialogue is vitally important to show the...
The fascinating aspect of act 2, scene 2 of Shakespeare's Macbeth is that very little actually happens in the scene other than dialogue between Macbeth and Lady Macbeth, but the scene is the emotional apex of the play.
It's true that the dialogue is vitally important to show the emotional contrast between Macbeth and Lady Macbeth and Macbeth's intense remorse and regret and near mental collapse at having murdered Duncan. Otherwise, the scene takes place almost entirely in the audience's imagination, as Shakespeare intends.
Lady Macbeth enters quietly, talking to herself, essentially giving the audience a glimpse of what she's done to prepare Duncan for his murder by Macbeth First she says that she got Duncan's guards drunk. It's not hard for the audience to envision that scene.
An owl shrieks, which frightens Lady Macbeth and the audience. The audience laughs at their own reaction, then Lady Macbeth brings them back to the scene. She left the doors to Duncan's rooms open for Macbeth, she says, and then she hears Macbeth cry out.
MACBETH. Who's there? What, ho! (2.2.11)
Lady Macbeth is concerned that Macbeth hasn't gone through with the murder, and she wishes that she had simply done it herself.
LADY MACBETH. Alack, I am afraid they have awaked
And ’tis not done. The attempt and not the deed
Confounds us (2.2.12–14).
She gives the audience another part of the picture: "I laid their daggers ready; / He could not miss ‘em" (2.2.14–15). Then she tells the audience something about herself that they absolutely never expected to know about her, which explains why she didn't kill Duncan herself:
LADY MACBETH. Had he not resembled
My father as he slept, I had done't (2.2.15–16).
That's a remarkable revelation, even if Lady Macbeth is only talking to herself.
The audience "sees" Lady Macbeth enter the room where Duncan is sleeping and look at Duncan lying asleep in his bed. They "see" her pause for a moment—perhaps wondering what she's thinking—then they "see" her lay the daggers next to Duncan and slip quietly out of the room, past the guards, and back to the room where she's standing now.
Macbeth enters, covered in blood, looking utterly distraught and broken, and he invites the audience to experience the horror of what he's just done.
MACBETH. This is a sorry sight (2.2.29).
That's all he says—five words—but that's all he needs to say for the audience to envision the entirety of the murder scene, and to compare that image with the image of the same room as Lady Macbeth described it just minutes before.
Now that the audience has "seen" what Lady Macbeth and Macbeth have seen, Shakespeare changes the focus of the experience and lets the audience "hear" what Macbeth heard while he stood horrified at the side of Duncan's bed, staring down at what he's done.
MACBETH. There's one did laugh in's sleep, and one cried,
...Me thought I heard a voice cry “Sleep no more!
Macbeth doth Murder sleep”
...Still it cried, “Sleep no more!” to all the house;
“Glamis hath murdered sleep, and therefore Cawdor
Shall sleep no more. Macbeth shall sleep no more" (2.2.30–31,46–47, 53–55).
Back to the present, Lady Macbeth sees the bloody daggers in Macbeth's hands, and thinks again that she should have done the murder herself and not to have trusted Macbeth to do it.
This gives Macbeth—and Shakespeare—an opportunity to remind the audience about the murder scene.
MACBETH. I'll go no more:
I am afraid to think what I have done;
Look on't again I dare not (2.2.64–66).
What could be so horrible that a battle-seasoned veteran soldier—a general of an army—who the day before met Macdonwald on the battlefield and "unseam'd him from the nave to the chaps, / And fix'd his head upon our battlements" (1..2.24–25) couldn't bear to look at?
Lady Macbeth gives the audience one more chance to envision the murder scene, and to add one more horror to it.
LADY MACBETH. If he do bleed,
I'll gild the faces of the grooms withal,
For it must seem their guilt (2.2.70–72).
Bloody Duncan lying dead in his bed isn't enough. Lady Macbeth finishes the painting of the murder scene for the audience by bloodying the guards with Duncan's blood.
Shakespeare doesn't show Duncan's death. He doesn't need to show it. Nothing that the actors could do on stage could compare to what each member of the audience envisions in their own mind.
After the horror of this scene, the "Porter scene" that follows comes as a welcome relief.