Why does Shakespeare switch between poetry and prose? For example, the witches speak in rhymed couplets of iambic tetrameter, the Porter speaks in prose, as does Lady Macbeth at times, and the bulk of the play is in iambic pentameter (this I get - it is the shifts that I'm not sure about). What effect on the listener/audience did Shakespeare intend with these shifts?
Personally, I think Shakespeare switches to poetry when he wants a character to say something really meaningful. To me, the most meaningful line in Macbeth is this one.
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.(30) (Act 5, Scene 5, p. 84)
This is beautiful. In fact, it is one of the most touching speeches in any of Shakespeare. It is Macbeth's one true, honest emotional scene. He realizes that his wife has died, and he sees his own mortality approaching.
He would also have characters shift in and out of verse and prose when they are in love (Claudio shifts to verse when he's describing Hero to Benedick, but other times uses straight prose), or when something is out of joint - something is not right with the world (that is usually a shift from verse to prose, but not necessarily).
Shakespeare used prose for the characters who are not of royal or noble nature and poetry/verse to indicate those who are.
As for Lady Macbeth, perhaps he is pointing out her fall from grace with the switch? Or her frame of mind...one of a base nature, not becoming to her status?
Interesting attempts have been made to explain Shakespeare's distinctive use of verse and prose; and of recent years there has been much discussion of the question "whether we are justified in supposing that Shakespeare was guided by any fixed principle in his employment of verse and prose, or whether he merely employed them, as fancy suggested, for the sake of variety and relief." In Hamlet five kinds of prose may be distinguished: (i) The prose of formal documents, as in Hamlet's three letters, II, ii, 120-124; IV, vi, 12-26; IV, vii, 43-47. In Shakespeare, prose is the usual medium for letters, proclamations, and other formal documents. (2) The prose of 'low life' and the speech of comic characters, as in the grave-digging scene, V, i. This is a development of the humorous prose found, for example, in Greene's comedies that deal with country life. (3) The colloquial prose of dialogue, as in the talk between Hamlet and the First Player, II, ii, 523-534, and in the conversation between Hamlet and Horatio, V, i. In both these passages, as in the grave-digging scene, the prose diction gives temporary emotional relief and prepares for the heightening of the dramatic pitch in the scenes which immediately follow. (4) The prose of abnormal mentality, as in the scenes where Hamlet plays the madman, or in IV, v, where Ophelia appears in her madness.(from http://www.shakespeare-online.com/plays/hamlet/hamletversehudson.html)
Shakespeare uses prose for lower-class characters, characters of an ustable frame of mind, and for formal documents such as letters and proclammations.