As preparation for his role, Sean Penn visited L.A. Goal, a center in Los Angeles for neurodivergent and intellectually disabled adults.
Dakota Fanning's character is named Lucy Diamond from The Beatles song "Lucy in the Sky with Diamonds." Michelle Pfeiffer's character's name is Rita Harrison, from the song "Lovely Rita," and the last name of Beatles member George Harrison. Sam, Lucy, and Sam's friends walking across the street with the balloons is a tribute to the case cover of "Abbey Road." Sean Penn wanted the soundtrack to be all Beatles music but was unable to get the rights, so he asked various artists to cover the Beatles songs he wanted to use. And in order for the re-recorded music to fit into the movie, each of the artists had to use the same tempo as the original Beatles songs.
Many lines or actions by Sean Penn's character, Sam, were improvised, such as the end of the scene at the new restaurant where Sam shouts, "Ask Big Bob!" or the dog scene. The producers liked many of these improvised scenes much better than the original book.