Shooting was completed on May 29, 1940, after which Sir Alfred Hitchcock made a visit to England. He returned on July 3 with the word that the Germans were expected to start bombing at any time. Ben Hecht was hurriedly called in and wrote the tacked-on final scene set at a London radio station. It was filmed on July 5, and the real-life bombing started on July 10, 1940.
Albert Bassermann (who played the Dutch diplomat Van Meer) couldn't speak a word of English and learned all his lines phonetically.
Producer Walter Wanger and Sir Alfred Hitchcock clashed repeatedly during shooting. Wanger kept wanting to have the script rewritten with every news story reporting changes in the European situation. Hitchcock, who hated making a movie without the script in absolutely final form before shooting began, pointed out that even if the movie were up-to-date at the time of shooting, it would be out-of-date by the time he finished post-production and it was ready for release.
In a 1972 interview on Dick Cavett Show (1968), Sir Alfred Hitchcock revealed that the plane crash scene was filmed by using footage shot from a stunt plane diving on the ocean, rear projected on rice paper in front of a cockpit set. Also, behind the rice paper were two chutes aimed at the cockpit's windshield connected to large tanks of water. With the press of a button at the right moment, water came crashing through the rice paper, into the plane simulating the plane crashing into the sea from the cockpit view.
When the plane crash sequence was shot, a special tub within the studio tank had to be built for Sir Herbert Marshall, who couldn't swim because he only had one leg (he'd lost a leg in combat in World War I).