When they pitched it, it didn't go anywhere. By recognizing the similarities with the book, they push it as an adaptation but a lot was lost.
The movie is very much a hybrid of the original pitch and the novel.
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Eh, I would argue you have to have some knowledge of the source material if you are going to make an adaptation, but sometimes you branch off of it as what works in a novel doesn't translate 100% to a screen. Some of the changes worked for Starship Troopers, others didn't. What it likely did best was have the same argument as the novel essentially. Both the movie and novel are argued if they are pro-fascist. You'll find people who believe that Starship Troopers was pro-service despite the obvious satire. The novel is more subtle with its message and it is debated on whether or not the author shared those views, but he also had work expressing opposing views.