Peter Sellers reportedly played a number of practical jokes on cast and crew during filming, including once calling Neil Simon up whilst imitating co-star Alec Guinness and demanding a re-write of a key scene in the middle of the night. Neither Guinness nor Simon was amused.
Immediately after completing the film, Peter Sellers was so convinced it was going to bomb, he convinced the producers to buy back his percentage share in the movie, thus depriving himself of a cut of the profits with the film when it went on to be a hit.
Neil Simon remained on the set to take care of re-writes, as he did with this picture's sequel, The Cheap Detective (1978). Simon took such a shine to Alec Guinness during the picture's production that he told him if he did not like anything in the film, he'd immediately rewrite it for him, but Guinness assured him it was great fun for him.
All of the detectives in the film are parodies of the work of three authors: Dashiell Hammett, whose Nick Charles and Sam Spade were the basis for Dick Charleston and Sam Diamond, respectively; Agatha Christie, whose Hercule Poirot and Miss Marple inspired Milo Perrier and Miss Marbles; Earl Derr Biggers Charlie Chan was the basis for Inspector Sidney Wang and his son.
In the television-edit of this movie - which contains footage not released theatrically or on home video - Keith McConnell and Richard Peel appear at the film's end as "Sherlock Holmes" and "Doctor Watson", respectively.
Myrna Loy was originally offered the part of Dora Charleston (a role that was a spoof of the character that she had played in the Thin Man movies) but she declined, later stating that "it would have been ridiculous to have Myrna Loy doing Myrna Loy". She also stated that she didn't want her "ass pinched by David Niven".
Originally Katharine Hepburn was meant to play a character called Dame Abigail Christian (a spin on Agatha Christie). Hepburn dropped out after hearing Myrna Loy would not do the film. The character was changed to Dame Abigail Christmas, and Estelle Winwood took the role. After numerous re-writes Estelle became Nurse Withers to a new character, Elsa Lanchester's Miss Jessica Marbles.
David Niven plays Dick Charleston, a role based on Nick Charles which was originated by William Powell in the Thin Man movies. This is the second time Niven has played a role originated by Powell, having also played the titular role in the remake of My Man Godfrey (1936).
During the first scene when Alec Guinness licks the stamps for the invitations, the stamps used were the 8 cent Dwight D. Eisenhower "No Dot", three-color stamps released in May 1971 and not the more popular 6 cent stamp released nearly a year earlier. First-class postage stamps were up to 13 cents by the time the movie was produced.
In the scene where each companion reports hearing gunshots to their respective sleuth, the visual of Miss Skeffington saying "Gunshots, Sam!" was removed from the final print of the movie; only the audio is heard. Both the audio and video of the sequence were included in the trailer for the film, however (as seen on the DVD release).
In addition to the spoofs of Hercule Poirot and Miss Marple, the film gives a nod to Agatha Christie's most famous story, And Then There Were None, which deals with a series of murders that follow a child's nursery rhyme. One of the lines of the rhyme is "Three little Indian boys walking in the Zoo / A big bear hugged one, and then there were two", and the corresponding murder is committed by dropping a bear-shaped marble clock onto the head of one of the victims. Here, four stone bear-shaped statues are dropped toward most of the guests in turn, hitting Marcel the chauffeur.
The trivia item below may give away important plot points.
Three memorable scenes from this movie ended up on the cutting-room floor: 1) The Charlestons narrowly avoid hitting Tess Skeffington; she's hiking back to her and Sam Diamond's car, with gasoline from a station five miles away. "Thank Heaven you saw me when you did," she says. Dick makes sure she's okay - "Keep to the side of the road," he advises her - then simply drives off and leaves her to walk. 2) Willie Wang - upon rejoining the others in the drawing room, after covering up Lionel Twain's corpse in the dining hall - has noticed something that none of the others did: Twain was clutching a note in his hand. Without bothering to read the note first, Willie gloats about how he is a better detective than his own father gives him credit for: "I'm sick and tired of being just your #3 adopted son. The $1 million goes to whoever solves the crime, and that could be me just as well as you!" Then Sam takes the note at gunpoint ("Better stand back, Tess; I don't want you to get hurt when the bullet comes out his other ear") and reads it ("'Please call dairy and stop deliveries of milk; Lionel Twain deceased.' So much for your clue, kid"). Willie sheepishly apologizes to Sidney. 3) Leaving Twain Manor at the end of the movie, the Wangs pass a vintage car en route to the same destination. It's driven by Sherlock Holmes and Doctor Watson. They ask directions, which Sidney gives them despite Willie's efforts to warn them it's a ripoff. Willie asks why his pop didn't tell them the truth. Sidney's reply: "Let idiots find out for themselves."