Fred Pohl talked about the movie in his Science Fiction: Studies in Film, way back when, and I was surprised to find it had been changed drastically from its original concept. Jerome Bixby (of Star Trek and Twilight Zone) and Richard Matheson* wrote the original script, which was more in the vein of a Jules Verne / "steampunk" adventure — "bronze laboratories, crystal instrumentation, that kind of thing", as Bixby put it. Then it was rewritten by Harry Kleiner, who took out all the fun stuff and put in Donald Pleasance — made it into "cops and robbers inside the human body".
The movie's interesting mostly for its effects work, which cost a ton of money and is fascinating today because it was all done in front of the camera, with the cast hanging from wires and with lighting and creative use of materials on the set instead of matte photography. It also featured an ending so dumb that Isaac Asimov's then-eleven-year-old daughter saw right through and out the other side.
The one innovation they plan to bring to the remake is to make it happen inside the body of an alien, or so it has been said. I think it would be best improved by making it happen inside the head of Ben Stein, so they could laser away the blood clot that turned him into a supporter of Intelligent Design.
Even stranger, by my book, is Robert Rodriguez picking up the rights to a live-action version of Ralph Bakshi's Fire and Ice. The original is mostly interesting for the sake of nostalgia value: Frazetta's design work looks better on the poster for the film than it does onscreen (it seems to consist mostly of putting people in as little to wear as possible), and the story's only intriguing if you're fourteen and still have Iron Maiden patches on your denim jacket. No offense to Iron Maiden fans.
* You know you've arrived when you don't even need to have credits attached to your name. I'd have to think I have to tell anyone reading this Matheson was the dude what wrote I Am Legend (the original story, you doof, not the lame movie), but hey, I guess it happens.