The trivia items below may give away important plot points.
The ending where the T-Rex saves the day was added when Steven Spielberg decided that it was the hero of the film.
In the original novel, John Hammond is killed by a small dinosaur called a Procompsognathid, a species which does not appear in this film. However, this death scene was resurrected and reworked for the sequel, The Lost World: Jurassic Park (1997).
In the original script, Gennaro and Malcolm were combined into one character, and Muldoon survived in the end. In the original book, Gennaro and Muldoon both survived, and Hammond and Malcolm died (though Malcolm returned in The Lost World: Jurassic Park (1997), explaining that "doctors did excellent work").
With every new draft of the script, there was a different set of survivors and a different set of characters dying. At various points during pre-production, Hammond, Malcolm, and Dr. Wu were going to die and Gennaro and Muldoon were going to live.
In one of the first drafts of the script, the character of Ian Malcolm (Jeff Goldblum) was not included. Instead, the character of Donald Gennaro (Martin Ferrero) is injured in the scene where the T-Rex attacks the cars.
Nedry's reasons for betraying Hammond are clearer in the novel, who is misled and never told anything about what goes on at Jurassic Park, forcing him to accept a bribe from a rival company.
Although Malcolm is sidelined for the latter half of the film, in the novel he is either kept silent or out of the story altogether.