Peter Ostrum and the other child actors have stayed close over the years, and regularly attend fan conventions together.
The reactions of the actors and actresses in some scenes are spontaneous: In the scene where Wonka limps out of his factory to greet the Golden Ticket winners, everyone's reaction is genuine. When the children first enter the Chocolate Room and see the candy gardens, their reactions are genuine. When filming the tunnel scene, the actors' reactions to Wonka's singing were genuine; Peter Ostrum, Jack Albertson, and Denise Nickerson were all terrified and had thought Gene Wilder had gone into a psychotic meltdown. In the scene where Wonka is screaming at Charlie and Grandpa Joe, Ostrum's and Albertson's reactions are real. Wilder actually wanted to tell Ostrum beforehand, but director Mel Stuart advised strongly against it, so as not to ruin the illusion of surprise.
When Gene Wilder died in 2016, Peter Ostrum changed his social media profile to "Former child actor, veterinarian, inherited a chocolate factory on August 29, 2016."
After reading the script, Gene Wilder said he would take the role of Willy Wonka under one condition: that he would be allowed to limp and then suddenly somersault in the scene when he first meets the children. When director Mel Stuart asked why, Wilder replied that having Wonka do this meant that "from that time on, no one will know if I'm lying or telling the truth." Stuart asked, "If I say no, you won't do the picture?" Wilder said, "I'm afraid that's the truth."
Ernst Ziegler, who played Grandpa George, was nearly blind (from poison gas in World War I), so he was instructed to look for a red light to guide him when his character was meant to be looking in a specific direction.