All the cast members had terrible experiences when it came to filming the scenes inside Walley World, where they rode all of the roller coasters and other rides. In the commentary, Chevy Chase mentions that many of the rides made him and the other cast members vomit, especially since they all had to ride them several times for each take. Dana Barron mentioned in the commentary that the coasters made her so sick, she had to take motion sickness pills and would pass out on nearby benches between takes. Finally, Anthony Michael Hall mentions that in the shots on the roller coaster where he looks scared, he wasn't acting, his fear in those shots was genuine.
In the Walley World scenes, Anthony Michael Hall is taller than Beverly D'Angelo; in previous scenes with her, they were approximately the same height. This was because Hall grew three inches after principal photography finished. The ending was re-shot four months later because the original ending failed with test audiences.
Many parts of the film were improvised including Clark's dance with his sandwich, Clark's send-off to Aunt Edna, and Rusty's chugging of the beer.
Imogene Coca originally turned down the part of Aunt Edna, because she did not think she could portray such a mean character. Even during filming, she was often concerned that she was being too mean to her fellow cast members.
Daisy Mabel (Cousin Eddie's tongueless daughter) was played by director Harold Ramis' daughter, Violet.
Lorenzo Salviati: Beverly D'Angelo's husband at the time is the musician playing the piano at the hotel bar during the scene where Clark meets up and chats with the Ferrari Girl (Christie Brinkley).
Harold Ramis: The voice of the recorded message of Marty Moose saying that Walley World is closed for maintenance.