The original appearances of Solid Snake, Big Boss, and Colonel Campbell were based on Mel Gibson, Sean Connery, and Richard Crenna, respectively.
Hideo Kojima, the game's designer, originally didn't have plans to do a Metal Gear sequel himself after the lackluster sales the game had in Japan. It was only after he became acquainted with one of the game designers behind Snake's Revenge (1990) (a sequel to Metal Gear made with the western market in mind, where Metal Gear was a surprise hit) that Kojima was inspired to develop a sequel himself.
The game's instruction manual features a fictional document outlining Snake's mission to the reader. The document was actually based on a similar mission sheet featured in the J.C Pollock novel Crossfire. Moreover, other aspects of the game (such as the romantic relationship between an American solder and a Czech skater) were also based on aspects in Pollock's book. J.C Pollock is reportedly one of Hideo Kojima's favorite authors.
During the game, Kessler tells Snake a rumor about Big Boss being given cybernetic implants by Dr. Petrovich after reportedly losing his limbs during the events of Metal Gear (1987). While certain fans interpreted this as a jab directed at Snake's Revenge (1990) (in which Big Boss is a cyborg), in reality this is a reference to Kojima's previous game Snatcher (1988). In Snatcher, the Dr. Petrovich Madnar character was the head of original snatcher project.
The first boss in the game, a member of the space ninja unit Black Color, was based on the titular characters from Timothy Zahn's novel, The Blackcollar. In Japanese, Blackcollar and Black Color are pronounced the same way.