Richard Burton and co-star Warren Mitchell were Royal Air Force cadets together at Oxford in 1944, where they knew one another and became friends. From 1944-47, when both were demobilized, they were stationed together at times in Canada and back in England.
Burt Lancaster was originally attached to play Alec Leamas.
David Lodge says in his memoirs he had disastrous interview with Martin Ritt for the part of Patmore.
In the room in East Germany where Leamas is interrogated by Fiedler, the wallpaper is of English design ("Marigold" by William Morris).
The phrase "Number 11 bus", which is heard a few times, is British slang for walking; your two legs look like the number 11.
Though playing a teenager, Claire Bloom was actually 34 at the time of filming.
When Richard Burton became a superstar, he insisted on casting his friends from his days at the Old Vic and West End (London's equivalent of Broadway). That is why Claire Bloom, who is clearly too old to be a teenager, was cast as Nan Perry. Other friends of Burton's cast in the film included the great stage actor Sir Michael Hordern and Robert Hardy.