Michael Crichton was a successful novelist, and his first foray into show business was writing the screenplay for Westworld, about a park where everything goes wrong. It flopped commercially but basically planted the seeds for him to try it again, but with dinosaurs. Spielberg directed the adaptation and then there was a rush to adapt a bunch of other stuff. He was also an executive producer for ER, as it was adapted from a pilot he wrote, based on his own experience from med school (he graduated with an MD but never practiced).