The neck frill in the movie was a complete fabrication on the part of Spielberg & Co, as was depicting the animal as much smaller than the real thing. In his original novel, Crichton described the animal according to the best info paleontology could currently provide, and then added his own speculative plot device that Dilophosaurus was poisonous. I think the producers did what they did more because someone thought it would "look cool" more than any other reason.
I've said it before and I'll say it again--I read Jurassic Park when it was first published, and I thought to myself, "This would be a great movie, if someone could only figure out how to animate the dinosaurs realistically." A couple of years later, when the movie came out, I was excited to see it. But after seeing the changes made to the story, I thought it would've been a much better film had they left out all of the human scenes and just shown the dinosaurs. And a couple of years later, riding the wave of renewed interest of JP, the BBC did just that, with their "Walking With Dinosaurs" series.