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.
During the famous moose scene, Dudley Moore kept ad-libbing so much that Stephen Elliott got enraged at having to do so many takes, he said, "Will you forget about the moose, for now". You can tell by Moore's reaction that it wasn't in the script. Director Steve Gordon halted filming and begged Moore to play the scene straight. That's why Elliot's tone changed so suddenly.
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.
Dudley Moore kept cracking up the cast and crew, so much so that 27 takes were needed to get one scene filmed.