Enforced Method Acting: According to IMDB, in the 1978 version, the directors did not tell the actress playing Nancy about the Twist Ending so her shriek of terror when Matthew points at her and begins to make the pod-scream is genuine.
The 1956 version ends with the cops catching the snatchers in the act, because the studio thought the original ending was too dark.
The 2007 version had a particularly bad case of this. The original director Oliver Hirschbiegel turned in a cut that was, like the previous versions, a psychological thriller. The studio wanted an action flick however, and hired the Wachowski brothers to refilm the entire third act.
English actress Dana Wynter plays Becky Driscoll in the original. They got around it in the script by saying the character had just returned to the U.S. after having lived in England for several years.
Another Englishwoman, Gabrielle Anwar, played the heroine in the 1993 version.
What Could Have Been: Legend has it that the 1956 version originally ended with the hero, Miles Bennell, on the highway screaming "You're next! You're next!" to all the passing motorists, leaving the audience on a cliffhanger, but the studio disliked this ending so it was changed to a more hopeful ending.