Anne Bancroft encouraged Mel Brooks to write the music as well as the lyrics for "Hope for the best, expect the worst" and subsequently at least one song for all of his movies. She was 'like an angel on his shoulder' when it came to songwriting.
Mel Brooks had problems with Yugoslavian extras who did not speak English. Some extras playing museum guards were to walk through the museum ringing a bell and shouting, "Closing time! Closing time!" The extras misunderstood and walked through the scene shouting "Cloakie Bye! Cloakie Bye!" Brooks decided "Cloakie Bye" was funnier, so he left it in the movie.
Mel Brooks originally offered the role of Ippolit to Gene Wilder. But Wilder wanted to play the role of Ostap instead. Brooks refused to cast Wilder because Ostap is described in the novel as "devilishly handsome". Wilder said that he was not offended by this, but still decided not to do the film.
When Ostap Bender is walking around Stargorod at the beginning of the movie, he turns from a street. There were an old and a new street sign hanging. The old one was "Czar Nicolaus II Avenue", the new one was "Marx, Engels, Lenin & XXXXXXX-Street". The censored name was Trotzkiy, a partner of Wladimir Lenin and Stalin, who was the head of the government of the USSR at the time the movie is set in (1927) was against him.
The trivia item below may give away important plot points.
Mel Brooks wanted to make a film which followed the original novel more closely than previous adaptations, but with one exception: he wanted the ending to be a "more sanitized happier ending". The novel's ending was much darker, having Ippolit murder Ostap shortly after discovering the twelfth and final chair in order to keep from splitting the diamonds with him.