Much of the laddish banter between Bodie and Doyle was improvised on-set by Collins and Shaw in order to entertain the crew, notably their conversation about Cowley in the Capri during Look After Annie (1978). However, these conversations proved to be so popular, that the editors left them in the finished versions, and they came to be regarded by many viewers as some of their favourite parts of the show.

For many years, Martin Shaw (Doyle) blocked repeat television showings of the series, disowning it almost immediately after it had finished. Although he gave his reason as not being able to negotiate fees for television repeats with program makers London Weekend Television, it was also alleged that he didn't want to be typecast with the "hard man" image the show portrayed him to have. He only eventually relented to repeat showings in the mid 1990s, when it was discreetly pointed out to him that the widow of Gordon Jackson (Cowley) could do with the income generated by repeat fees.

During the 1984 Libyan Embassy siege, crowds of protesters surrounded the British Embassy in Libya, chanting, "Down with CI5!", unaware that it was a fictional organization, and the name of the British Secret Service is actually MI5.

Episode Klansmen (1977), in which black people are harassed by a group that has modelled itself on the Ku Klux Klan, was banned and never shown in the UK because of its racial content.

According to his official character history, Bodie's full name is William Andrew Phillip Bodie, and he is originally from Liverpool, and of Irish descent. His mother was a royalist, and named him after various Princes. He ran away to join the merchant Navy as a teenager, and became involved in gun-running to Africa, then jumped ship in order to become a mercenary (throughout the series, Bodie's mercenary and arms dealing connections would come in extremely useful). He would later return to the UK, and join the Parachute Regiment, then the S.A.S., seeing action in the Northern Ireland conflict before being recruited into CI5. In "Mixed Doubles", Bodie states that he joined CI5 for the money, although it's clear Doyle doesn't believe him. Bodie's past is almost identical to that of the character of "Milke Gambit", played by Gareth Hunt in "The New Avengers" (1976), although Gambit would rise to officer rank in the Army, while Bodie was a Sergeant. Bodie and Gambit were heavily influenced by the background of the actors who played them, Lewis Collins served as a paratrooper, and Gareth Hunt running away to sea as a teenager.