The 1951 film was scripted by Edmund H. North, and based on Harry Bates' short story "Return of the Master", a story which still reads well today.
But did you know that the early 1980s nearly saw the return of Klaatu, in a sequel written by Ray Bradbury?
Bradbury's screen treatment for Twentieth Century-Fox was entitled The Evening Of The Second Day, and was drafted in March 1981, with revisions completed in September 1981. Bradbury was initially opposed to the idea of such a sequel. According to Starlog magazine, he told the studio "Don't do it. The original film is so beautiful. Why don't you blow it up on larger film stock and re-release it, because nobody wants to see a sequel."
The studio bosses replied, "Yes, but we want you to do it."
Bradbury described his plot for the film like this:
The return of Klaatu. He comes back under refrigeration because he has been dead, semi-dead. His body is encased in ice so you wouldn't see him very well and we wouldn't have to change characters.
Klaatu's daughter brings him back and they land on Earth at Cape Canaveral on Christmas Eve. They signify their arrival, proving how powerful they are by lighting all the towers all the way down Cape Canaveral. Oh wow! I thought it would be terrific if we could show you all the towers lit like Christmas trees on Christmas Eve. They're offering a promise, aren't they? A gift to the world. They stay around for a while and at the story's end, on New Year's Eve, they take off for the universe. Of course, that's a celebration also - and along the way, there's the usual Bradbury optimism.
I liked some of the ideas I had. They were very visual. Of course, you have to out-metaphor the other film. And what is there left to do [laughs].
Starlog magazine, Sept 1981, p23
A few days ago I remarked that Bradbury wrote little that was related to Christmas, so this film would have been a notable exception. It's certainly typical of Bradbury that he would be seduced by a strong central image, and curious that he should have chosen a Christmas setting for the return of Klaatu, especially since some critics have emphasised the Christ-like attributes and behaviour of the original character.
Would Bradbury's treatment have made a good film? It is is difficult to tell. I have no idea which director (if any) was attached to the project, nor whether this would be a "major motion picture" or just a cheap cash-in sequel. In 1981 Hollywood was still trying to come to terms with the Star Wars phenomenon, with lots of attempts to cash in on George Lucas' unexpected box-office smash. For every Blade Runner or ET, there were a dozen cheap and embarrassing Star Wars knock-offs.
In any case, Bradbury's screen treatment didn't progress to a screenplay, and the project faded away. Just another Bradbury film project that might have been.