The screenplay originally called for only a few trick photography shots of Hayley Mills in scenes with herself. The bulk of the movie was to be shot using a body double. After producer Walt Disney saw how seamless the processed shots were, he ordered the script reconfigured to include more of the visual effect with her playing both twins in the same scenes.
In the dance party scene where the layer cake falls off the beak of the totem pole and drops onto Miss Inch's face, writer and director David Swift originally wanted to cut the shot of the cake fall. But after producer Walt Disney saw the rushes, he told Swift to leave the shot in, saying it would be the biggest laugh in the movie.
In the shot where Mitch Evers trips on the chair, Brian Keith fell so hard he cracked a rib, but kept on doing the scene.
The title song was performed by Tommy Sands and Annette Funicello. They were on the lot shooting Babes in Toyland (1961).
Susan Henning took on the role as Hayley Mills' body double for many of the twin shots in the movie. As part of her contract, she signed away her rights to be credited. At the wrap party, Walt Disney presented her with a small statue of Donald Duck, called "The Duckster" in recognition of the "best unseen performance on film." Ms. Henning subsequently appeared in the Elvis Presley vehicle Live a Little, Love a Little (1968).