Susan Henning took on the role as Hayley Mills' body double for several of the twin shots in the film. 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." Only 2 other Ducksters have ever been awarded. To her credit, Ms. Henning did later star with Elvis Presley.
The screenplay originally called for only a few trick photography shots of Hayley Mills in scenes with herself; the bulk of the film was to be shot using a body double. When Walt Disney saw how seamless the processed shots were, he ordered the script reconfigured to include more of the special effect.
For the scene where Maureen O'Hara and Charles Ruggles are speaking in the bedroom, Ruggles didn't have any place to put the ashes of his cigarette. Director David Swift mentioned someone he knew who put the ashes in his hand, and Ruggles asked for permission to use this. He did this in so casual a way that the ashes would be on the floor seconds later.
The actor who plays "Mr. Eaglewood", the director of the boy's camp, is Frank De Vol. He was not only a gifted character actor, but he was also a composer. He did the soundtracks for many great films and TV shows.