Almost every movie adaptation of a book involves a certain amount of poetic license. What works in a novel, doesn't always work in a book, so changes of plot and detail are necessary.
Babylon A.D. starring Vin Diesel, based upon the book "Babylon Babies" (originally written in French) which I read (in translation) over the winter break, was released in August 2008. The movie is receiving renewed hype as it comes to video, despite being one of the biggest box office flops of 2008. It reputedly cost $70 billion to make, yet earned only $23 million in theaters.
I haven't seen it and I'm not excited to do. Braving spoilers to read the plot synopsis on line made one thing clear. This wasn't an "adapted" screenplay, this was a "loosely inspired by" screenplay. While the original was merely B grade dime store novel material, with a number of good ideas that were poorly executed, the adaptation sounds atrocious.
The name, powers, significance and entire life story of the heroine have been radically altered. The main female supporting character for the heroine now works for someone else and bears almost no resemblance to the original except her name. The main male supporting character for the leading man ended up on the cutting room floor along with the leading man's back story. Several action sequences with major plot elements are inserted where there was only an uneventful airplane trip in the original. The end of the book plays out in a Montreal heavily fleshed out with local color in the novel, which is removed to New York City in the movie.
None of the warring factions in the original, whose complex politics and betrayals drive the novel, survive the adaptation, and a new "off screen" character is designed to provide something else to drive the plot instead. The most climactic battle of the movie that parallels a battle in the novel, whose cause is central to the novel's meaning, is started by someone else for a different reason.
In short, the plot, characters, themes, setting, mood and pacing of the novel are so different from the movie that the movie might, with just one or two more name changes, not even qualify as a derivative work for copyright purposes.
There is nothing wrong with writing an original science fiction screen play. People do it all the time. But, if you are going to so profoundly change your source, why even bother doing an adaptation in the first place? The normal premise behind an adapted screenplay is that you can reduce risk by working with proven material. But, as this box office flop demonstrated, if you stray from the proven material, you haven't do anything to reduce your risk.
As a footnote, the only reason that I read the novel was because it has been misclassified by the library as a romance. Neither the movie nor the novel fit that bill. The novel is an elaborate suspense story in the style of John Le Carre, with supernatural/pseudo-science elements incorporated, while the movie is a vapid action flick with post-apocalyptic elements.