In one of his handbills, the fraudulent duke bills himself as “world-renowned Shakespearian tragedian, Garrick the Younger, of Drury Lane, London.” This is an allusion to the very famous David Garrick, an extremely well-regarded actor from the eighteenth century who had a theater at Drury Lane in London.
The duke also alludes to the famous balcony scene in Romeo and Juliet when he mentions:
Juliet’s in a balcony, enjoying the moonlight before she goes to bed, and she’s got on her night-gown and her ruffled nightcap.
Some of the humor comes in the fact that the original Juliet would not have worn a ruffled Victorian nightcap.
When the duke tries to piece together Hamlet's "to be or not to be" soliloquy, he inserts an allusion to a different play, Macbeth, when he says "till Birnam Wood do come to Dunsinane."
A Macbeth allusion, again misplaced, enters the same Hamlet soliloquy in "the poor cat i’ the adage." The poor cat in the adage liked fish but did not like getting its paw wet catching them. This alludes to Macbeth wanting to be king but not wanting to do the dirty work to get the crown.
In his Shakespeare playbill, the duke also alludes to David Kean, yet another famous British Shakespearean actor who had already died.
Huck describes the Grangerford house as "whitewashed on the outside." Although he may not be conscious of what he is saying, Biblically literate readers would have quickly understood this as an allusion to both the books of Matthew and Luke in which Jesus accuses the Pharisees of being cups that are white and clean on the outside but not so on the inside. This is an apt description of the feuding, slave-owning Grangerfords who go to church armed with rifles and have paintings, a piano, and beautiful furniture—as if they are gracious and noble—while owning other human beings.
Many of the allusions in the novel are humorous, playing on the idea that the reader is more knowledgeable than the characters.