John Belushi's performance in the cafeteria scene was entirely improvised. When he began piling food on his tray, director John Landis urged the camera operator to "stay with him." The infamous "I'm a zit" gag was also improvised, and the reaction from the cast is completely genuine.
Reputedly one of Donald Sutherland's personal favorites of all his films. He described working in it as the funniest experience he had on a set.
DeWayne Jessie's performance as "Otis Day" was so successful that he legally changed his name to Otis Day and subsequently toured and recorded with "Otis Day and the Knights."
Donald Sutherland was so convinced of the movie's lack of potential that, when offered a percent of the gross or a flat fee of $75,000 for his three days' work, he took the upfront payment. Had he taken the gross percentage, he would have been worth an additional $3-4 million.
Verna Bloom said that her scene with Dean Wormer, where she is drunk, and he is on the phone with the Mayor, was completely improvised, because John Landis was unhappy with the dialogue written into the script.