Compare Othello's level of dictation and use of antithesis and metaphor in lines 450–453 with his diction in lines 453–455 and 478–481. The above lines are from act 3, scene 3.

I will do the best I can with this, and I am glad you supplied more information. First, diction is the style of speech used by a character, including use of such poetic devices as metaphor and antithesis. Diction can be poetic, heavy with literary devices, or prosaic, more like ordinary speech.
Metaphor is comparison not using the words "like" or "and." Antithesis is putting opposite ideas together.
In the first set of lines, the dialogue is as follows:

IAGO
And may, but how? How satisfied, my lord?
Would you, the supervisor, grossly gape on,
Behold her topped?



OTHELLO
  Death and damnation! Oh!

Iago uses poetic diction in these lines. In other words, he is not using everyday, conversational speech. He doesn't simply say, "but how is this going to work? Are you going to watch them have sex?" Instead, Iago uses alliteration, a poetic device which puts words that begin with same consonant side by side, saying "grossly gape." He also, more importantly, uses imagery, which is description we can experience with the five senses of sight, sound, touch, taste, or smell: we can picture Othello "grossly gaping" or staring hard with his mouth hanging open at his wife having sex with Cassio. Iago uses imagery again, when he pictures Desdemona "topped" or Cassio on top of her having sex. Iago is trying to make these images as vivid and animalistic as possible so that Othello will imagine them fully and think they are real. Iago wants to inflame Othello and make him as angry as possible, which is why he uses poetic diction. And it works. Othello's language, emphasized by an exclamation point and cursing (damnation), shows how very upset he is becoming. He doesn't just want Cassio and Desdemona dead, but damned.

The second set of lines is as follows:


It were a tedious difficulty, I think,
To bring them to that prospect. Damn them then,
If ever mortal eyes do see them bolster
More than their own! What then?



How then?
What shall I say? Where’s satisfaction?
It is impossible you should see this,
Were they as prime as goats, as hot as monkeys . . . .

[and Othello responds] Give me a living reason she’s disloyal.


In the above passage, Iago is saying, look, how on earth are we going to make you an eye-witness to this affair? (Iago knows, of course, he can't, because he is lying about it.) Here, Iago's diction becomes less poetic, but we see him mirroring Othello's speech by using "damn" and apparently expressing emotion with "more than their own!" as indicated by the exclamation point. Iago goes on to use similes, comparisons using "like" or "as," to compare the lovers to goats or monkeys having sex, again trying to inflame Othello with the animal imagery of lust.

At the end of this passage, Othello's diction is prosaic, or ordinary, simply asking for a reason why Desdemona would be disloyal. He appears to be calming down and using his reason.

Third passage:


And then, sir, would he gripe and wring my hand,
Cry “O sweet creature!” and then kiss me hard,
As if he plucked up kisses by the roots
That grew upon my lips, lay his leg
Over my thigh, and sigh, and kiss, and then
Cry “Cursed fate that gave thee to the Moor!”




In the third passage, as he witnesses Othello using his reason at the end of the second passage, Iago realizes he better ratchet up his diction again and whip up Othello's emotions. Since he can't produce the couple in bed, he tells a lie. He uses imagery to describe Cassio asleep in bed beside him. Iago says Cassio had a dream in which he thought Iago was Desdemona. He paints a vivid descriptive sight and sound picture of Cassio calling out the words "sweet creature," kissing Iago "hard," throwing his leg over Iago's thigh, and cursing Othello in his sleep. We can both see and hear this happening.


Iago uses another simile, comparing Cassio's kisses to pulling up a plant by the roots—he seems to be implying that Cassio is kissing his dream Desdemona very passionately, as if drawing her mouth into his or putting his tongue deeply into her mouth.

This heightened diction again works, as we can hear in Othello's distressed response of "monstrous . . . !"

Overall, these passages show that when Iago uses heightened diction, particularly imagery, to paint a picture, he manages to make Othello very upset—even though everything Iago says is a lie. It sounds real, however, and Othello responds as if it is real. This happens in the first and third passage. In the second passage, Iago loses his grip for a moment, and starts getting sidetracked into more ordinary diction. In this case, Othello's response becomes more rational. The rational response is the antithesis or opposite of what Iago wants.