Here's an interesting phenomenon:
1. Author writes bestselling novel.
2. Bestselling novel becomes blockbuster movie.
3. Author writes a sequel--but to the movie rather than the original novel.
I first noticed this pattern with Alistair MacLean's sequel to The Guns of Navarone. That novel, published in 1957, ends with the three stalwart male heroes--Mallory, Miller, and Andrea--safely aboard a British naval vessel. When the 1961 film version was produced, certain changes were made, including the introduction of a female character, Maria, who serves as a potential love interest for Andrea.
MacLean got around to writing the sequel, Force 10 From Navarone, in 1968. It picks up right where the original story left off--except now, only Mallory and Miller are on board that British destroyer. Why? Because Andrea has returned to Navarone to marry Maria--a character that wasn't even in the original novel!
It's not clear why MacLean did this, especially since it really only impacts the first chapter of the book. Perhaps he was trying to make things as simple as possible for Hollywood and smooth the way for another Navarone film. If so, his plan backfired--the film version of Force 10 arrived a full decade after the novel. It featured an entirely different cast, and suffered a significantly different critical reception and box office gross, than its classic predecessor. (How bad was it? If you happen to meet Harrison Ford at a cocktail party, this isn't the film you want to bring up.)
Whatever his motive, MacLean's choice seems to be tacit acknowledgment of a film's power to supplant its literary source material in the popular imagination.
Brian Garfield was very clear about his motives for writing a sequel to his 1972 novel Death Wish. That book ends with urban vigilante Paul Benjamin gunning down criminals in New York City while the police turn a blind eye to his activities. The infamous 1974 film version ends with the vigilante, now named Paul Kersey, run out of New York by the police and preparing to set up shop in Chicago. Garfield's 1975 sequel, Death Sentence, seems to follow on the movie's ending--the action picks up with vigilante Paul Whatever-His-Name-Is in Chicago. But Garfield felt compelled to write the second book because he was dismayed by the film's apparent glorification of vigilantism. Discussing Death Sentence in an interview with the website Novel Journey, Garfield said: "The novel, which I wrote years ago as a sort of penance for the movie version of Death Wish, attempts to demonstrate in dramatic form that vigilantism is not a solution--it's a problem, and tends to destroy those who attempt it." If writing Death Sentence was penance for Garfield, enduring the four Death Wish film sequels must have been hell.
Like Garfield, David Morrell wrote sequels to his 1972 novel First Blood to protect the integrity of his original creation. Morrell introduced the iconic character Rambo in that novel--and killed him off at the end of it. But while Rambo died in the book, he survived in the 1982 film version. Rambo really exploded in popularity with the movie's sequel, Rambo: First Blood Part II, released in 1985, which was based on an original screenplay rather than a novel. Morrell stepped in to provide novelizations for that film and for a third movie, Rambo III (1988).
So how did Morrell deal with the ticklish fact that Rambo died in the original novel when he was writing the follow-up books? With a terse author's note: "In my novel First Blood, Rambo died. In the films, he lives." If only resurrection were that easy!
In a Foreword to a reissue of First Blood, Morrell elaborated:
"I wasn't involved with the films. However, I did write a novelization for each of the sequels in an effort to supply the characterization that they omitted. I felt that it was important to remind readers of what the novel's Rambo had been about."
A particularly illuminating example is Paul Gallico's 1978 sequel Beyond the Poseidon Adventure. Your first clue to the genesis of this book: it's dedicated to "Master of Disaster" Irwin Allen, who produced the 1972 film version of The Poseidon Adventure. In a surprisingly frank Foreword, Gallico describes it as "a book brought about wholly by modern times. It is a sequel, not to the original novel, but to the film made from that novel." The problem Gallico faced: The original book ends with the Poseidon sinking, while in the big screen version the capsized liner is still afloat as the end credits roll. "How fortunate!" Gallico wrote. "For had not Irwin Allen decided upon this ending to the picture and with the Poseidon at the bottom of the sea no sequel would have been possible. The novel herewith produced takes the surviving characters as delineated in the motion picture and carries on with some of them as they were, looked, and behaved in the film. . . .The main thing to remember is that this book is a sequel to the film and everything that was in that film."
Gallico essentially admits that the second book was a marketing strategy cooked up between himself and Allen: "This is where both producer and writer find themselves in these days when hit movies are followed by sequels using the same characters and sequel novels flower at the same time," he wrote. "Ever since the rise of the paperback the film has needed the novel, the novel the film. Each aids in the campaigns of publicity and promotion."
Gallico made one prescient prediction: "It is almost certain that, just as in the first case, the film Beyond the Poseidon Adventure will make changes from the novel to the detriment, one hopes, of neither." The film that ultimately resulted, released in 1979, largely abandoned Gallico's plot and characters. It was more of a movie disaster than a disaster movie, and sank a lot faster than the Poseidon did.
All of these examples are from decades ago, so either this isn't happening anymore or I need to catch up on my reading. If there are other (more recent) examples out there, I'd be interested to know about them.