It still amazes me how well the VFX in Jurassic Park holds up after all these years. Industrial Light and Magic is the team that amazingly brought the dinosaurs to life on the big screen. We have a fascinating featurette that you need to watch that focuses on bringing the live-action elements together with the VFX elements. It's really cool to get a before and after look at all the movie magic that had to happen to bring this classic film to life for us.
Here's a little trivia that gives some backstory on what Steven Spielberg needed to do to make this movie work.
Steven Spielberg didn’t want people to be constantly reminded that what they’re seeing is CGI, but real, full-blooded dinosaurs, starting with the Brachiosaur scene, where Spielberg was keen on the dinosaur interacting with the background, and would offer suggestions to the animators on how to make it better. The second scene done in that same vein was the Gallimimus scene, which made use of 25 animated individual Gallimimuses. Geometric shapes represented them initially and were choreographed onto the scene. Spielberg needed complete freedom to convey the energy of the scene, so worked with Dennis Muren to shoot it, because he wanted to move the camera and not lock it down everytime a Gallimimus came into frame. The scene was shot gradually with Sam Neill, Joseph Mazzello and Ariana Richards running through a field by themselves. A grid was placed over the ground as a frame to chart the movement of the camera by computer, using what looked like golf balls whenever an actor looked somewhere. The dinosaurs were than added later.
The T-Rex chasing the Jeep was the most difficult scene to animate. Steve ‘Spaz’ Williams had to do research because there’s no frame of reference for a running animal of that size; it took two months to figure out how to get it to run, for instance. He would run the sequence backwards to see all the mistakes. They were also able to use the computer to add little details to authenticate the scene, e.g. the T-Rex running through puddles of water and leaving splashes, etc. The splashing was filmed individually and then the computer added it to the T-Rex’s footsteps.
I hope you like this! Thanks to /Film for the tip!