At one point during the production, Liza Minnelli was supposed to board a bus in front of Bergdorf's on Fifth Avenue. When a real bus came along, she boarded it thinking it was the "movie bus". Not until she was halfway down the block did she realize her blunder when she looked back and saw the whole crew cracking up.
Dudley Moore was said to have based his performance partly on Peter Cook, whose excessive drinking had soured his and Moore's comedic partnership in the 1970s.
Dudley Moore kept cracking up the cast and crew, so much so that 27 takes were needed to get one scene filmed.
Apparently, writer and director Steve Gordon had originally wanted to cast George Segal in the lead role of Arthur. After the box-office success of 10 (1979), however, Dudley Moore replaced Segal. This movie was actually the second time that Moore replaced Segal. Segal was originally cast in the lead role in 10 (1979), but Segal apparently walked off the set shortly after filming began. Reportedly, this was rumored because Julie Andrews' role had been built up. Apparently, after 10 (1979) had been released, Segal, when once asked if he had seen the movie, allegedly replied with a finger gesture.
Sir John Gielgud turned down the role of Hobson several times, finally accepting it only because the salary he was offered was too good to pass up.