According to Robert Loggia, on the day they filmed the famous Walking Piano scene at FAO Schwarz, he and Tom Hanks noticed that doubles dressed like them were on hand just in case they could not do the dance moves correctly. It became their goal to do the entire number without the aid of the doubles. They succeeded.
To give Tom Hanks an idea of how a 13-year-old would behave, director Penny Marshall filmed each "grown-up" scene with David Moscow (Young Josh) playing Hanks's part, and then Hanks copied Moscow's behavior. Hanks would go on to do something similar for Forrest Gump (1994), when he would spend time with Michael Conner Humphreys (Young Forrest) and imitate his Southern accent to prepare for the part.
Tom Hanks was the first choice for the role of Josh Baskin but was unavailable due to scheduling conflicts with Joe Friday (1987) and Rien que pour rire (1988). Robert De Niro was then offered it, and the writers began to retool the script for a more urban 13-year-old, but he was rejected because his $6 million salary demand was too high. Hanks then became available and accepted it for $2 million. David Moscow originally auditioned not for young Josh but for Billy because he didn't look like De Niro. When Hanks was given the role, Moscow was brought back in to audition for young Josh.
Penny Marshall became the first female director to ever direct a movie that grossed more than $100 million at the box office with this movie.
According to Monica Rushton, Jared Rushton (Billy), David Moscow (Young 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.