Theater has always been a collaborative art. In Shakespeare's time, a repertory company could not expect a playwright to write in a vacuum. The nature of the schedule, in which a new play could be commissioned weekly, required playwrights to collaborate. English playwrights in the late 16th and early 17th centuries freely borrowed material from one another and shared criticisms and edits. Hamlet, like the other great works attributed to Shakespeare, definitely presents Shakespeare's work, but also showcases many contributions by actors, managers, prompters, and so forth, who all knew what parts of a play to leave in or take out.
Like the Greeks, Elizabethan and Jacobean audiences attended the theater to watch plays they had seen often or which were based on stories as familiar to them as their own family histories. Accordingly, Shakespeare based his Hamlet on a popular Scandivian saga that had existed for at least a hundred years in one form or another, and which actors all over Europe had performed in earlier manifestations as early as the 1550's.
Newington Butts featured the Lord Chamberlain's Men in Hamlet, an earlier revenge play, directed by Henslowe, on June 9, 1594. Scholars commonly call this Hamlet the Ur-Hamlet and believe the author to be Shakespeare's brilliant contemporary Thomas Kyd. Neither a copy of the Ur-Hamlet nor concrete evidence that Kyd actually wrote the play exist, but the story of Hamlet does bear a strong resemblance to Kyd's masterpiece The Spanish Tragedy, which many scholars believe to be a perfected version of the Ur-Hamlet.
