How did Shakespeare loosen his verse line, and when does he drop into prose?

Shakespeare's characters speak in verse by default, and the verse itself changed over his career: it began tidy and stiff and became loose and quick. Prose is the other gear. It is mostly saved for different kinds of speakers.

The standard line is blank verse: unrhymed, ten syllables, a stress on every second syllable. In his early plays, sentences tend to start, pause and finish at the end of lines, which is lovely but risks monotony. Once he had mastered the form, he began to interrupt and vary its flow, and that is where the verse in plays such as Julius Caesar and Hamlet gets its new power.

The key trick is enjambment: a sentence that runs over the end of a line instead of stopping there. Lines that stop at the end are called end-stopped, and end-stopping is more frequent in early Shakespeare. As his style developed, enjambment increased so reliably that scholars such as Goswin König and A. C. Bradley used its frequency to estimate dates for undated works. In his last plays he also often ended lines on an unstressed syllable, a feminine ending.

Prose is the exception, not the rule: many passages are in prose, but he almost always wrote a large proportion of each play in iambic pentameter. In Romeo and Juliet he saves prose most often for common people, and now and then Mercutio.