Director Don Siegel asked Charles Bronson to shave his trademark mustache off for this movie. Bronson replied, "No mustache, no Bronson." Siegel said in his book 'A Siegel Film: An Autobiography', "I felt that as much as Bronson wore a heavy mustache in Russia, it would help his disguise if he had no mustache when he arrives in Canada. However he didn't want to shave it off."
The name of the poem used as the code spoken over the telephone to activate sleepers was 'Stopping by Woods on a Snowy Evening' by Robert Frost. This poem was originally published in 1923 in his collection titled 'New Hampshire'. The exact lines from the poem heard in the movie each time were: "The woods are lovely, dark and deep. But I have promises to keep, And miles to go before I sleep, And miles to go before I sleep."
Reportedly, during filming, star Charles Bronson didn't want to kiss actress Lee Remick in the scene at airport. Bronson said that "When my wife meets me at an airport, we never kiss." Director Don Siegel asked Remick to kiss him anyway and Remick replied, "But, Don, I don't dare. He's liable to hit me!" The airport greeting scene went ahead and made Don's day.
When Borzov (Charles Bronson) encounters Immigration on the Canadian border, the agent asks about his citizenship and birthplace, to which he responds, "American, from Pennsylvania", Bronson's true birthplace.
The Russian news media agency 'Izvestia' was highly critical of this American cold war thriller movie and this became international news.