The diction of a play is talking about the language used within the play.

Verse and Prose , Prose could be called “normal language” – it is what we use in everyday  speech. … Verse can also be called poetry – it tends to have a regular rhythm, and is divided into “stanzas” rather than paragraphs. ‘Low’ or comic characters generally speak in prose in Elizabethan and Jacobean drama. Traditionally, scenes of madness were also written in prose. Shakespeare obeys these conventions in King Lear, but you will notice that prose is used on other occasions too. Sometimes scenes contain both verse and prose. Verse can sometimes rhyme.

Lear is the king in this novel therefore he is of the highest power and we would expect him to speak in Verse.Lear uses the language device of switching between verse and pros to show his madness affecting him and signifies the disruption in his mind as the play carries on.In the storm scene we see Lear switch between verse and pros quiet frequently this shows the chaos around him and madness affecting him. QUOTES …

Edgar is Gloucesters legitimate son born into upper class and proper , he begins the novel speaking in verse because he is living his normal upper class life but as the novel continues and events occur that affect him and he turns into poor tom his speech changes to Prose

” Who gives any thing to poor Tom? whom the foul
fiend hath led through fire and through flame, and
through ford and whirlipool e’er bog and quagmire;
that hath laid knives under his pillow, and halters
in his pew; set ratsbane by his porridge; made film
proud of heart, to ride on a bay trotting-horse over
four-inched bridges, to course his own shadow for a
traitor. Bless thy five wits! Tom’s a-cold,–O, do
de, do de, do de. Bless thee from whirlwinds,
star-blasting, and taking! Do poor Tom some
charity, whom the foul fiend vexes: there could I
have him now,–and there,–and there again, and there. ” – Edgar. This quote Edgar is speaking in prose , this is because he is disguised as poor tom and acting poor and crazy to conceal his real identity.When Edgar reemerges as himself he goes back to speaking in verse

Gloucester is another character who switches between verse and prose through out the play.Gloucester is of higher class but in act 1 he switches between verse and prose regularly