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.
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 The Professionals: 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 favorite parts of the show.
Episode The Professionals: 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.
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.
Collins and Shaw first worked together in The New Avengers episode The New Avengers: Obsession (1977), Shaw as a disgruntled R.A.F. Officer, and Collins playing a mercenary, not unlike Bodie. At one point, Collins' character even remarks to Shaw that they make a good team and should work together again some day. When Anthony Andrews dropped out of playing Bodie on this show, the show's creator's remembered Collins, and the friction that had existed between him and Shaw on-set, creating dramatic tension, and earning him the role.
In the unofficial pilot, The Professionals: Old Dog with New Tricks (1978) Bodie used a Smith & Wesson .38 Special revolver, and Doyle a Walther PPK automatic pistol. By the time of The Professionals: Private Madness, Public Danger (1977), they have both swapped to Browning 9mm semi-automatics, the standard British military handgun of the time. By the third season, Bodie is using a Smith & Wesson .357 Magnum, and Doyle a Walther 9mm. In The Professionals: Mixed Doubles (1980), both Agents use Smith & Wesson .44 Magnum revolvers. Cowley uses a Colt .38 revolver throughout the series. All three use a variety of small arms, notably Armalite and FN FAL rifles and Uzi, Ingram, and Sterling submachine guns. In The Professionals: Wild Justice (1980), Bodie and Doyle used prototype versions of the British Army's SA80 assault rifle, which hadn't even been trialled by the S.A.S. at the time.
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.
Martin Shaw publicly disliked the series, describing his character as a "violent moppet". By contrast, Lewis Collins revelled in his tough guy image, becoming a black belt in martial arts, and joining the Territorial Army as a paratrooper. Allegedly, he also attempted to join one of the Territorial Army's Special Air Service (S.A.S.) Regiments, but was rejected as being too famous for their covert role. He would go on to play an S.A.S. Officer in The Final Option (1982) and Code Name: Wild Geese (1984) (Codename Wildgeese). The actors' contrasting attitudes influenced the writing for their characters, Bodie very much being written as a tougher, more ruthless individual with few qualms about his job, while Doyle is more thoughtful and troubled by the violence he has seen.
Ray Doyle's background was that he was born in Derby before moving to London at a young age. He describes himself as a "right tearaway" as a teenager, "cutting up another kid when he was just a kid" himself. He joined the Metropolitan Police to "get some discipline", and was their pistol shooting champion, rising to the rank of detective. He served in the forces' anti-drugs squad, and in his spare time, founded a martial arts club for local youths. In The Professionals: Mixed Doubles (1980), he states that he was glad to join CI5, as the line between what he was doing, and what the criminals were doing, was becoming dangerously thin. In real-life, the Metropolitan Police was wracked by numerous corruption scandals in the late 1970s.
In 2018, the UK's ITV4 were still showing the un-restored old telecine versions of the episodes, although the restored versions had been since released on blu-ray and dvd a few years previously. However, ITV4 had started to show the original unrestored versions in 12:9 aspect ratio cropped top and bottom to fit a 16:9 screen. Presumably, the restored masters will eventually be available to broadcast once the home entertainment licence for the restored version lapses.
We are never told exactly how large CI5 is, or what exactly its powers and responsibilities are. However, we do meet Agents with three-digit callsigns, and as Cowley is a Major, it would suggest that the organization has between one hundred and one hundred twenty operatives. Their activities seem to be largely restricted to the UK, although on at least one occasion, we see Bodie conduct a spy-swap on the Cold War border between East and West Germany. At least thirteen CI5 Agents are killed in the line of duty during the six year period of the series.