Why do you think Shakespeare occasionally fiddles with the rules of iambic pentameter? What effect does this have on a reader or listener who is paying close attention?

Shakespeare had acquired such a mastery of language and metre that he often disregarded the rules which earlier poets, and he himself in his earlier works, had carefully observed. Shakespeare almost without exception puts prose rather than verse into the mouths of the insane, and Lady Macbeth's somnambulism is meant by him to be regarded as a symptom of her mental disorder.

Metre of Macbeth I know Metre of Macbeth