Honor Blackman was cast at short notice replacing a performer who dropped out.
Senta Berger was first cast.
Brigitte Bardot was an early choice to portray Tracy (Mrs. James Bond) in On Her Majesty's Secret Service. She was unable to do so because she was shooting this movie with Sean Connery, on a temporary retirement from 007.
Henry Fonda was originally cast as the lead.
According to her memoirs Ingrid Pitt was considered for the role of Lady Julia but turned it down to pursue a (successful) casting role in Where Eagles Dare.
Final film of Peter van Eyck.
This is the film that Sean Connery made while taking a break from Bond movies. Due in part to the difficult time Connery had in Japan while filming You Only Live Twice the Bond producers released Connery from his contract (otherwise he would have been required to star in On Her Majesty's Secret Service).