The emergent lifeform was a CGI animation created by Amblin Entertainment, the production company that belongs to Steven Spielberg. The MacPherson Nebula was an old creation of Visual Effects Supervisor Dan Curry, who had been experimenting with laser light on plastic surfaces.
No new sets were needed for this episode. The train set was a reuse from Bram Stoker's Dracula (1992), and Paramount's standing New York City street set was used for Keystone City. The staircase the crew walk up belong to New York City Subway's 42nd Street/Times Square Station, according to the sign on the back.
At the beginning of the episode, Data is on the holodeck playing the role of Prospero and being coached by Picard. The following year (1995), Patrick Stewart played the role of Prospero on Broadway.
The holodeck characters are seen occasionally cutting paper dolls (i.e. cutting folded paper to make a long chain of cut-out figures). The term "cutting paper dolls" is also slang for boxers who are "punch-drunk", and is a subtle clue that things are not right with the characters' higher reasoning systems.