According to Blake Edwards in the DVD boxed set documentary, a week into filming, Peter Sellers disappeared. When he returned, from an apparent holiday, Edwards was ready to kill him until Sellers told him of a peculiar hotel manager he met who had a funny French accent which Sellers was to give to the Clouseau character. Thus was born Clouseau's trademark odd pronunciations such as "beump" for bump and "meuths" for moths.
Elke Sommer had already signed for two German films when she landed the role of Maria. She used $100,000 of her fee to buy her way out of that commitment so she could sign a three-year contract with MGM.
George Sanders told the crew that each time anyone would break up in laughter at Peter Sellers because of his pronunciation, there would be a fine of £5 be put in a kitty. At the end of the production, the kitty had built up to £200, which were given to a poor farmer and his old horses who lived not far from the production sets.
Although they had worked well together on La pantera rosa (1963), the improvisational nature of Uno sparo nel buio (1964) led to many arguments between Peter Sellers and Blake Edwards. At the end of filming, they both agreed never to work together again. This wouldn't last. They would ultimately re-team four years later for Hollywood Party (1968) and, subsequently, several Pink Panther sequels.