To give star Tom Hanks an idea of how a 12 year-old would behave, director Penny Marshall filmed each "grown-up" scene with David Moscow (Young Josh) playing Hanks' part, who then copied Moscow's behavior.
According to actor Robert Loggia, on the day they filmed the famous keyboard scene at F.A.O. Schwarz, he and Tom Hanks noticed that doubles dressed like them were on hand just in case the two could not do the dance moves correctly. It became their goal to do the entire keyboard number without the aid of the doubles. They succeeded.
The 'Walking Piano', first spotted by the filmmakers at F.A.O. Schwarz toy store, was 6.5 feet long and played only one octave. So the piano was too small to play the notes director Penny Marshall needed (the script was written that Tom Hanks and Robert Loggia would play 'Heart and Soul' on the piano). So she contacted Remo Saraceni, the creator of the 'Walking Piano', and said she needed one built large enough to accommodate the dancing feet of two grown men. So the obliging Saraceni made a 16-foot long, full three-octave piano wide enough for the scene.
Tom Hanks was the first choice to play Josh Baskin but was unavailable due to scheduling conflicts with the films Dragnet (1987) and Punchline (1988). Robert De Niro was then offered the lead role, and was rejected because his salary demand ($6 million) was too high. Hanks then became available and accepted the lead role for $2 million. David Moscow was originally cast not as young Josh, but as Billy, since he didn't look like De Niro. When Hanks was given the role, Moscow was recast as young Josh.
When Penny Marshall got the script, nobody was interested in doing the movie. It was only when Robert De Niro announced he wanted the part of Josh that the script received attention from people wanting to do it.
In preparation for the role, Tom Hanks met David Moscow and studied videotapes of him to see how he behaved and spoke. Hanks also felt that Moscow should just be himself so that the Josh Baskin character persona would be that of a real 12-year-old.
According to Monica Rushton, Jared Rushton (Billy), David Moscow (little Josh) and Tom Hanks (big Josh) were put in a room with a bunch of toys to play with. Having silly string, they tried to use it to gross each other out and that is how the silly string scene between Hanks and Rushton appeared in the movie.
In the film, Josh comes up with the brilliant idea of robots that transform into prehistoric, giant insects. His description is nearly identical to the Insecticons, a group of robots from The Transformers toy line. The Insecticon toys were released three years before the film.
For the two missing social security numbers on Joshua's employment application, he gives his age (12). Because he stammers, saying "Oh" twice, the interviewer thinks he gave a three-digit response, 0-1-2 (pronounced "oh-one-two"). Combined with the six-digit locker combination Billy gave him, this would appear to be a full social security number.
The scenes in Josh's neighborhood were filmed in Cliffside Park, New Jersey. The amusement park scene in the beginning of the movie where the wish was made was filmed at Ross Dock Picnic Area on the Hudson in Fort Lee. It is not an amusement park but was set up as one. The George Washington Bridge can be seen clearly in some of the shots. The scene at the end where he finds the machine were filmed on the pier at Rye Playland, in New York's Westchester County.
The "Walking Piano" used in the film's trademark scene was created by an Italian inventor named Remo Saraceni. Mr. Saraceni's many musical inventions grace children's organizations worldwide. As of 2008, his famous Walking Piano is currently being implemented into a piano instructional game called Piano Wizard made by Allegro Multimedia.
The Shimmy Shimmy Coco Pop rap was Tom Hanks's idea. According to Tom Hanks in an interview with Jonathan Ross, Hanks stated that the rap was something his son had learned at Summer Camp and Tom Hanks came up with the rap for the film and made up the words to the rap.
It was rumored that an alternate ending has been filmed in which Susan had used the Zoltar machine and had made herself a little girl again and is seen sitting in Josh's class and that this ending was used in the CBS/FOX video release in New Zealand. However, no alternate ending had been filmed. This is however the exact ending of the film, 14 Going on 30 (1988) which was released around the same time and has a very similar plot. Possibly the source of this rumour.
The offices of an actual advertising agency on 23rd Street in the Chelsea district in New York City was used for the scenes involving the fictitious MacMillan Toys Company, which were very hard on both the film crew and the actual workers of the agency. The location fee of $25,000 was donated to the American Craft Museum by the agency and Twentieth Century-Fox.
In the party scene, Josh tries caviar, which he hates. He spits it out & wipes his tongue with a napkin. In You've Got Mail (1998), Tom Hanks responds to Meg Ryans comment "that caviar is a garnish!" by scooping up all the remaining caviar for himself.
Barry Sonnenfeld worked as director of photography for this movie. During the sleepover scene, where Josh shows Susan his New York apartment and toys, Patrice Rushen's "Forget Me Nots" can be heard playing in the background. Will Smith would later sample this song for his successful Men in Black (1997) theme, written for a hit franchise directed by none other than Barry Sonnenfeld.
The trivia items below may give away important plot points.
The costumers of the movie took special care to ensure that Susan's character realistically transitions from uptight businesswoman at the beginning of the film, to a sweet, almost girl-like persona that she is at the end of the film. Notice that her wardrobe and hair gradually scene by scene become less and less adult (hair goes from pinned up to loose and with schoolgirl hand bands, clothes from tight suits to loose girlish angora separates and young skirts, shoes from heels to flats), and more more that of a schoolgirl. "Susan" may turn down the offer to become a little girl again, but we are left with the clear impression that thanks to her relationship with the Tom Hanks "adult" character, she has found her inner child.