Movie adaptations have a way of fundamentally altering their original works in ways that aren't always apparent. A good actor, in particular, conveys an impression of literary characters that can't be undone. It's hard to read, say, The Godfather without hearing Marlon Brando speak Vito Corleone's dialogue or doing the same with Fight Club and envisioning Tyler Durden as anyone but Brad Pitt. Those changes can drastically affect public perception of the work, sometimes for decades or more.
Case in point: Jurassic Park and, more specifically, the character of Ian Malcolm. Author Michael Crichton's "chaotician" wasn't particularly notable in the original novel, especially in comparison to what he became in Steven Spielberg's beloved 1993 adaptation. In this case, the movie character was so strong that Crichton brought him back in the books.
Malcolm is one of the consultants of the titular park, brought in to help John Hammond construct it "safely." Malcolm's expertise on chaos theory leads him to believe that the endeavor will end in disaster. Of course, it does, as the dinosaurs break out and the staff and visitors are suddenly placed in dire peril. Malcolm panics and tries to run when the T. Rex breaks out of its paddock, only to be attacked and gravely injured. He spends the remainder of the book gradually growing worse until seemingly expiring at the end.
Malcolm's primary function in the novel was plot exposition, or more precisely, tech exposition. As with many of his works, Crichton based Jurassic Park around several abstract theoretical concepts, including chaos theory. Malcolm was there to spell out the specifics in an organic fashion to make sure the reader was caught up on all the ideas. Beyond that, he was largely superfluous, and since the story entailed prehistoric carnivores running amuck, he made an easy victim to sideline early on, leaving room to focus on Dr. Grant and the children still lost in the park.
That works well on the printed page when dialogue and exposition are presented in the same manner as action and visual description. However, in the movies, it can lead to dull material, as an actor basically stands around and talks. Furthermore, Malcolm's craven behavior in the book -- running away while children are in danger -- makes it harder to sympathize with him, despite the fact that he's right about the dangers of the park. Spielberg and, specifically, screenwriter David Koepp thus made the necessary changes to lend the character some weight.
That started by casting Jeff Goldblum in the part: a talented actor with a deliberately quirky onscreen persona and a reputation for eccentric-yet-memorable performances. The film then gave him a lounge lizard's fashion sense and the self-regard of an academic "rock star," as well as a wandering eye that tempered his charisma with real character flaws. With that shading and depth in place, Koepp could then eliminate his most distasteful quality -- abandoning children to danger -- leaving a far more memorable anti-hero behind. As such, the "bloodsucking lawyer" Gennaro gets the honor of being eaten by the T-Rex in Malcolm's place.
And it worked, to the point where Malcolm felt like an indispensable part of the story. Crichton himself restored the character to life in the sequel novel The Lost World, and by the time Goldblum returned for the second movie, his character was there to stay. None of it would have happened if Spielberg and Koepp had stuck purely to the novel, and Jurassic Park would have been a little diminished as a result.