But even by the very relaxed standards of the day, Shakespeare was invigoratingly wayward.

From Shakespeare The World as Stage by Bill Bryson.  Chapter 5 page 102.

In the rush to entertain masses of people repeatedly, the rules of presentation became exceedingly elastic. In classical drama plays were strictly either comedies or tragedies. Elizabethan playwrights refused to be bound by such rigidities and put comic scenes in the darkest tragedies—the porter answering a late knock in Macbeth, for instance. In so doing they invented comic relief. In classical drama only three performers were permitted to speak in a given scene, and no character was allowed to talk to himself or the audience—so there were no soliloquies and no asides. These are features without which Shakespeare could never have become Shakespeare. Above all, plays before Shakespeare’s day were traditionally governed by what were known as “the unities”—the three principles of dramatic presentation derived from Aristotle’s Poetics, which demanded that dramas should take place in one day, in one place, and have a single plot. Shakespeare was happy enough to observe this restriction when it suited him (as in The Comedy of Errors), but he could never have written Hamlet or Macbeth or any of his other greatest works if he had felt strictly bound by it.

Other theatrical conventions were unformed or just emerging. The division of plays into acts and scenes—something else strictly regulated in classical drama—was yet unsettled in England. Ben Jonson inserted a new scene and scene number each time an additional character stepped onstage, however briefly or inconsequentially, but others did not use scene divisions at all. For the audience it mattered little, since action was continuous. The practice of pausing between acts didn’t begin until plays moved indoors, late in Shakespeare’s career, and it became necessary to break from time to time to trim the lights.

Almost the only “rule” in London theater that was still faithfully followed was the one we now call, for convenience, the law of reentry, which stated that a character couldn’t exit from one scene and reappear immediately in the next. He had rather to go away for a while. Thus, in Richard II, John of Gaunt makes an abrupt and awkward departure purely to be able to take part in a vital scene that follows. Why this rule out of all the many was faithfully observed has never, as far as I can make out, been satisfactorily explained.

But even by the very relaxed standards of the day, Shakespeare was invigoratingly wayward. He could, as in Julius Caesar, kill off the title character with the play not half done (though Caesar does come back later, briefly, as a ghost). He could write a play like Hamlet, where the main character speaks 1,495 lines (nearly as many as the number spoken by all the characters combined in The Comedy of Errors) but disappears for unnervingly long stretches—for nearly half an hour at one point. He constantly teased reality, reminding the audience that they were not in the real world but in a theater, as when he asked in Henry V, “Can this cockpit hold the vastie fields of France?” or implored the audience in Henry VI, Part 3 to “eke out our performance with your mind.