Barney's car is a 1964 Aston Martin DB5 convertible. Only 1,059 DB5s were made, with only 123 being convertibles. In 2020, one of these cars could easily be worth $1M or more.
Sandra Dee was cast as the female lead but was replaced. During pre-production in France, director Jack Smight had doubts she could play the role of the "British girl" the script called for, so she was paid off and Susannah York was cast instead, with the agreement of Jack L. Warner.
This was Warren Beatty's second consecutive picture filmed in London, though unlike Promise Her Anything (1966), much of it actually takes place in that city.
Clive Revill, who plays Susannah York's father, is only nine years older.
Harry Dominion's profile bears a strong resemblance to Napoleon. Early in the film Angel McGinnis tells Barney that she has always had a thing for Napoleon and so when Dominion walks across the poker room she greets him with "Vivre Le'Emperor" and Dominion responds by placing his hand across his chest inside his jacket in the style of Napoleon to return the gesture.