Dominic West was originally cast to play Kim Philby, but had to withdraw from the project because of Covid delays and his ongoing commitment to The Crown (2016).
Elliot attributes Kim's success as a double agent to his exceptional ability to befriend people. The real Philby said, in a speech given to the KGB, that the main reason he went undetected for so long was due to classism: the British establishment was, in his view, unable to conceive that someone like him, born and raised in the elite class of empire, would turn against that system and aid the communists.
The series was filmed in in London and Romania, finishing in Spring 2022. With Bucharest standing in for Beirut, Moscow, Vienna and Berlin.
With the exception of Lily Thomas, virtually all the major characters are real people: Kim Philby, Nicholas Elliot, James Jesus Angleton. Guy Burgess, Anthony Blunt, etc.
The plot line with James Jesus Angleton attempting to run Philby as a triple agent in Moscow is fictional, but Philby's anger at the way the Soviets quickly rounded up the American agents may be inspired by the case of Aldrich Ames, CIA officer who began spying for the Soviets in the 1980s. Unlike Philby, Ames was not a Communist and instead betrayed his country for money. Ames gave the Soviets the names of numerous Soviet citizens who were spying for the Americans and the Soviets began rapidly rounding them up. Ames' Soviet handlers actually apologized to him, because this rapid series of arrests lead the CIA to conclude that they had a leak, thus endangering Ames.