The Duke and the Dauphin rehearse Romeo and Juliet, however, they also prepare monologues from other plays.
The Dauphin (the King) is going to play Juliet and the Duke will play Romeo. The rehearsals, though they are impressive to Huck and Jim, are recognizable to the reader as highly imperfect versions of the Shakespearean texts. The Duke claims to have memorized the texts correctly, but presents considerable inaccuracies.
These misreadings and poor versions of the plays serve a comic role in the novel, presenting yet another layer of deception (and self-deception) in the characters of the Duke and the Dauphin.