There were many plot changes in the film version. Secondary characters were dropped, the cast of dinosaurs was made smaller, and many dinosaurs were changed in the scenes that were left intact.
Here are some changes with the characters:
Ian Malcolm: At the end of the book, Ian is taken back to the lodge where he is declared dead despite medical attention (his injuries are caused by a tyrannosaurus). He returns in 'The Lost World' revealing that he had simply come near to death. However, in the film, he escapes with a broken leg and is perfectly healthy in the film sequel 'Jurassic Park: The Lost World'. Ian is played by Jeff Goldblum in both Jurassic Park films.
John Hammond: John Hammond is the creator of Jurassic Park. In the book, he is mean-spirited and purely interested in profit, lacking interest in the technicalities of genetic engineering, whereas in the film, he is a sympathetic grandfather. He is killed in the book, so therefore does not appear in the book sequel. In the film, he survives and makes a brief appearance in the film sequel. He also regrets his creation of the park in the film, but planned to create a new one in the book. John Hammond is portrayed by Richard Attenborough in both films.
Dr. Alan Grant: Dr. Grant escapes with T-rex and raptor-inflicted wounds in the book, whereas in the film he escapes barely wounded. Dr. Alan Grant is played by Sam Neill.
Ellie Sattler: Ellie Sattler is engaged to a doctor in the book, whereas in the film she is in a relationship with Dr. Grant. Ellie is played by Laura Dern in the film.
