For many years actor Martin Shaw (Doyle) blocked repeat TV 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 TV repeats with programme 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.
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.
In the unofficial pilot The Professionals: Old Dog with New Tricks (1978) Bodie uses a Smith and 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 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 use prototype versions of the British Army's SA80 assault rifle which hadn't even been trialled by the SAS at the time.
Much of the laddish banter between Bodie and Doyle was actually 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 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.
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 SAS, 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 although Gambit would rise to officer rank in the army whilst Bodie was a Sergeant. Both Bodie and Gambit were heavily influenced by the background of the actors who played them, Lewis Collins serving as a paratrooper and Gareth Hunt running away to sea as a teenager.
Collins and Shaw first worked together in The New Avengers episode The New Avengers: Obsession (1977), Shaw as a disgruntled RAF 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 in The Professionals 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.
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 organisation and the name of the British Secret Service is actually MI5.
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 SAS officer in the films 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 whilst 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.
George Cowley fought as a volunteer for the Republicans in the Spanish Civil War. He was wounded in the leg during an air attack by German bombers which is why he limps during season one. Presumably he had surgery for the bullet to be removed by season 2 as the limp disappears. After Spain Cowley would go on to serve as a commando in World War 2 and the Korean War eventually reaching the rank of Major before joining MI6 and then being appointed as head of MI5 before leaving to create CI5. According to Bodie and Doyle in The Professionals: Mixed Doubles (1980) Cowley appears to be religious. His old nickname in MI6 was 'Morris' based on the contemporary car the Morris Cowley. CI5 agents occasionally refer to him as 'The Cow'. His nickname for Bodie and Doyle is 'The Bisto Kids' based on a pair of cartoon characters used to advertise gravy. His favourite drink is malt Scotch.
We are never told exactly how large CI5 is or what exactly its powers and responsibilities are. However we do meet agents with 3-digit callsigns and as Cowley is a Major it would suggest that the organisation has between 100-120 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 13 CI5 agents are killed in the line of duty during the six year period of the series.
A number of episodes feature regular English background actors who appeared in other programmes. Some famous faces too: Ruby Wax, Pamela Stephenson, Stephen Rea, Ian McDiarmid and briefly - Pierce Brosnan (Series 4 episode 8 "Blood Sports" as a radio man).