Alvin Kernan, The Playwright as Magician. New Haven: Yale University Press, 1979. pp. 102–103.
From time to time this submerged or latent theater in Hamlet becomes almost overt. It is close to the surface in Hamlet’s pretense of madness, the “antic disposition” he puts on to protect himself and prevent his antagonists from plucking out the heart of his mystery. It is even closer to the surface when Hamlet enters his mother’s room and holds up, side by side, the pictures of the two kings, Old Hamlet and Claudius, and proceeds to describe for her the true nature of the choice she has made, presenting truth by means of a show. Similarly, when he leaps into the open grave at Ophelia’s funeral, ranting in high heroic terms, he is acting out for Laertes, and perhaps for himself as well, the folly of excessive, melodramatic expressions of grief.
Almost all of Shakespeare’s Hamlet can be understood as a play about acting and the theater. For example, in Act 1, Hamlet adopts a pretense of madness that he uses to protect himself and prevent his antagonists from discovering his mission to revenge his father’s murder. He also presents truth by means of a show when he compares the portraits of Gertrude’s two husbands in order to describe for her the true nature of the choice she has made. And when he leaps in Ophelia’s open grave ranting in high heroic terms, Hamlet is acting out the folly of excessive, melodramatic expressions of grief.
"This passage, in content and structure, is taken wholesale from the source. Although the writer has rewritten much of the paragraph, and fewer phrases are lifted verbatim from the source, this is a clear example of plagiarism. Inserting even short phrases from the source into a new sentence still requires placing quotations around the borrowed words and citing the author. If even one phrase is good enough to borrow, it must be properly set off by quotation marks. In the case above, if the writer had rewritten the entire paragraph and used only Alvin Kernan’s phrase “high heroic terms” without properly quoting and acknowledging its source, the writer would have plagiarized."
Source: Princeton University, http://www.princeton.edu/pr/pub/integrity/pages/plagiarism/