Novelizations of films usually add additional background material not found in the original work to flesh out the story, due to the fact that novels are generally longer than film screenplays. Similarly, material from the screenplay or filmed scenes that are cut from the final production may still be present in a novelization, often because the novel is completed for publication while the film is still being edited for release. The practice was particularly popular for successful films and television series before the advent of home video, which gave people the ability to record programs or buy commercial releases for repeated viewing.
A novel as a tie-in to any successful major film release is considered an important marketing strategy. If a film is based on a novel, the novel is generally reissued with a cover based on the film's poster (sometimes with pictures from the film included) but if it is an original screenplay or based on a novella or short story a novelization is often commissioned to fill the marketing niche. Novelizations have been published since at least the 1920s; for example, the 1928 film The Fleet's In! starring Clara Bow was novelized by Russell Holman.
A similar practice is an adaptation of a film in comic book form, essentially an illustrated novelization. The art in a comics adaptation can vary from original perspectives to directly copying still scenes from the film, though issues of actors' personality rights may prevent exact likeness of film characters. The majority of Marvel Comics' Super Special series featured film adaptations; DC Comics customarily prints adaptations as stand-alone, one-shot issues. Sometimes a comic book adaptation can lead to an ongoing licensed series by the same publisher; examples of these include Marvel's Star Wars series based on the Original trilogy and DC's various series based on the first six Star Trek films, both of which featured stories set during the "gaps" between the films.
It has recently become common for authors who write novelizations to write original novel sequels in between novelizations. The novels Halo: The Fall of Reach and Halo: First Strike, which serve as prequels and sequels to the video game Halo: Combat Evolved, were written before and after the novelization of the game. Author Greg Cox wrote the original novel Underworld: Blood Enemy after writing the novelization of Underworld and before writing the novelization of the sequel film Underworld: Evolution. Also, while novelizing the Resident Evil video games, author S. D. Perry wrote original novels that took place between the novelizations. With the rise in recent years of films based on comic book properties, the publishers of the adapted properties have also started producing prequel comics as supplemental material, set in the canon of the films rather than that of the source material.
see also: Randall D. Larson, "Films Into Books", Lanham, MD: Scarecrow Press, 1995.