Gregory Peck found the stunts particularly difficult because of an old leg injury due to horseback riding. So during the cornstalk scene Peck had to keep telling Sophia Loren to slow down because it looked as if she were rescuing him and not the other way around.
The part of David Pollock was originally written for Cary Grant. When Gregory Peck had trouble getting the humor right in a line, he'd smile and tell director Stanley Donen "Remember, I'm no Cary Grant."
As many critics noted (not always approvingly), Stanley Donen filmed this thriller in an uncharacteristically flamboyant style, using bizarre camera angles and eccentric visual compositions throughout. He later admitted that he had never felt that the screenplay was quite right (many writers worked on it, and it was rumored to be still being re-worked during shooting), so he had given the film an unusual look to disguise its shortcomings. He had had to start filming before he was quite ready, in order to accommodate the busy schedules of Gregory Peck and Sophia Loren.
An interesting note is that this movie is the first in the career of Stunt Man, Vic Armstrong. Armstrong is listed in the Guinness Book of World Records as the World's Most Prolific Stunt Man. Vic Armstrong began his career as Gregory Peck's Stunt Horse Riding Double on this movie.
The "Pierre Marton" who is part-credited with the screenplay is, in fact, Peter Stone, who had scripted Stanley Donen's previous film, "Charade". He was to use this pseudonym twice more in his career. "Marton" was the surname of his stepfather, George Marton, whilst the French word "pierre" can be either a name, meaning "Peter", or a noun, meaning "stone".
The sequence on the railway viaduct was filmed on the Crumlin Viaduct in South Wales between the closure of the railway to traffic on 13 June 1964 and the demolition of the structure, which began in June 1966. It was built in 1853-57, and at 200ft was the highest in the British Isles (and the third highest in the world).