During the post-production process of the film, Paramount kept Stan Winston's Kothoga creature under wraps much like Universal had done with Jurassic Park (1993), with no one permitted to talk about the production or the creature itself.
The character of D'agosta is actually a melding of two characters from the book. The characters combined are D'agosta and FBI agent Pendergast which is the main character in the sequel book and many books after.
The preparation of the fictional elixir given to John Whitney by the Amazon tribesmen, and the ceremony surrounding it, strongly parallels the real-life consumption of ayahuasca, a psychoactive tea used religiously by numerous Amazon tribes.
The two sniffer dogs used to track down the creature in the sewer system are called "Castor" and "Pollux", sharing their names with the Troy Brothers from John Woo's Face/Off (1997), which in turn are named after the Dioscuri from Roman/Greek mythology.
The appearance of the Kothoga creature was purposely left till the last half of the film because Stan Winston's creature was not completed in time for the scenes requiring the visual and physical presence of it on set. It was also one of the reasons the film was delayed until the winter of 1996/97.