Paleontologist Alan Grant had served as a consulting expert for the Hammond Foundation, supposedly helping them set up a museum exhibit on baby dinosaurs. The foundation hires him to visit Jurassic Park and inspect it for safety. Grant has decades of experience in the field. He is one of two characters voicing concern about the park's safety and disproving InGen's claims. (Dr. Malcolm is the other.) Grant is also exceptionally responsible. When the tour group is disrupted and scattered, he takes responsibility for Hammond's grandchildren and keeps them safe, guiding them through multiple dinosaur attacks. At the climax of the novel, he kills several raptors through trickery, and at the novel's end, he insists on inspecting the raptor nests himself.
An outspoken and idiosyncratic mathematician, Malcolm visits the park to examine it for safety, and to make sure the dinosaurs are contained. Along with Grant, Malcolm is one of two visitors to the park who directly challenge John Hammond's claims about the park's safety. Throughout the novel Malcolm can see things others can't. His knowledge of mathematical models shows him that the dinosaurs have escaped and are breeding. Malcolm serves as a kind of prophet in the novel, predicting doom before it arrives, and explaining the limits of science, even as he's dying. Malcolm dies after being bitten and thrown by a Tyrannosaurus rex. Crichton said the work of American physicist Heinz Pagels inspired the character of Malcolm.
John Hammond insists on his vision of dinosaurs and the park, declaring that the structure is safe and the dinosaurs are under control. The novel reveals Hammond as arrogant and blind to the truth: he rejects concerns voiced by his hired experts, Grant and Malcolm, about the park's risks. He sees the dinosaurs as something he can control, and the park as a highly profitable investment. Dinosaurs kill and eat Hammond late in the novel.
Tim Murphy and his sister Lex visit Jurassic Park because their parents are divorcing. Tim is 11 and loves dinosaurs. Grant is his hero. Tim and Lex join the tour and nearly die several times when dinosaurs attack. Tim helps save the park by restarting the computer and monitoring systems.
Donald Gennaro is an ambivalent character. His initial motives seem largely selfish: he wants to protect his company's investment in the park. He also chooses to stay with the sick stegosaur because he's attracted to Ellie Sattler. However, he helps start the generator and inspects the raptor nest with Grant, showing a fundamental responsibility and a fair degree of bravery.
Sattler joins the Jurassic Park tour to check for problems with the park's plants while Grant looks into the facility's animal population. In addition to spotting toxic ferns, she discovers the toxins that have felled one of the park's stegosaurs.
Nedry, while an expert with computers, is unhappy working for John Hammond. He is kept in the dark about many of Jurassic Park's data programming needs and feels blamed for bugs that end up in the system. Offered a bribe by InGen's rival, Biosyn, he accepts and hacks the park's systems to enable the theft.