The scene is dominated by Lady Macbeth which makes it markedly different from the rest of the play. Much of it is in blank verse as Lady Macbeth recounts her husband's meeting with the witches. It is a short, stark scene which is a shocking introduction to Macbeth's 'dearest partner of greatness'.
The audience is taken aback by her incantations to the spirits and her desire to be the embodiment of evil-
...unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty!
As Macbeth has been portrayed as the 'worthy Thane', ruthless in battle we are surprised to see that his wife is the driving force here-
Leave all the rest to me.
Shakespeare's purpose in doing this seems to be to show us the determination of Lady Macbeth for her husband to take the throne. Also, we see that Lady Macbeth in calling upon evil spirits is requesting a pathway of immorality; a descision which we will see her punished for later.
First, let me assume that we are talking about Act 1, Scene, 5 because it is certainly different and singularly important. It is the scene in which we meet the other half of a deadly duo of Macbeth and his Lady.
This scene begins as Lady Macbeth is reading aloud a letter written to her by her husband, Macbeth. It tells her about the meeting he had with the three witches and what they had promised to him, most notably that he will be king in the future. After she finishes the letter, she has a little conversation with her self in which she shows how excitied she is about becoming the queen of Scotland and how she will get Macbeth to kill Duncan so that he can become the King.
Just then a messenger rushes in and tells her that Duncan is coming to her castle for dinner that very night. Lady Macbeth is sure this is part of the witches' prophesy and begins to ready her body and all her dark being for murder.
Macbeth then enters. Filled with excitement, Lady Macbeth greets her hubby and tells him of her nefarious plans for the evening... not exactly dinner and a movie.
No, there is no other scene in the play like this. It is used to introduce us to a very unusual, strong-willed, cold-blooded and malevolent individual.
There is a very important reason why Shakespeare changes his style of presentation in this scene of Macbeth. He uses much more blank verse, the reading of a letter and the whole scene reads more like a soliloquoy. One of the reasons William Shakespeare does this is because we, the audience, need to hear Lady Macbeth's evil thoughts from her own lips so that she damns herself in our eyes. He lets it be seen that there is no duress, no undue pressure - in other words no excuse for what is to follow. We need to see that just because many women were under the subjugation of their husbands, it is not so with the Macbeths. There needs to be no mistake about it. So we hear Lady Macbeth put voice to her own inner thoughts. In its own way, it is a powerful scene full of dark import and impact.
If scene v of Act I isn't the scene in question, then it must certainly be scene v of Act III. This is the scene with Hecate, the goddess of witches. The style here is in rhyming couplets, unlike any other scene of the play, and because of this, many think that Shakespeare did not write it.
Why is it done this way? One can only speculate. One, the rhymes definitely add to the mystical powers of Hecate, and two, they develop her seductive and beguiling charm. If you notice, the witches always speak in couplets. This is all part of their allure.