Real World article
(written from a Production point of view)

Referenced in the final revised draft script of the Star Trek: The Original Series episode "The Conscience of the King", Marcellus is a character in William Shakespeare's play Hamlet.

Marcellus was mentioned once in the final revised draft of the script for "The Conscience of the King" (dated 8 September 1966). The script referred to Marcellus as being portrayed by a member of the Karidian Company of Players during a performance of Hamlet aboard the USS Enterprise, in the starship's theater. The actor playing Marcellus walked offstage, with Horatio, just as Captain James T. Kirk entered the backstage area.

"Marcellus" was also originally to have been the name of the character ultimately called Claudius Marcus, in TOS: "Bread and Circuses".