To maintain the pure fantasy of it, there were strict rules about what could and could not be shown in an episode: 1. No "uniformed policemen". 2. No "colored people". 3. No blood. 4. No dead women. 5. No blatant sex.
In one episode during the fourth season, Steed receives a Christmas card from Cathy Gale and it is postmarked "Fort Knox". This is in reference to Honor Blackman's appearance as Pussy Galore in the James Bond film Goldfinger (1964).
Series writer Brian Clemens noted in an interview the sexual chemistry that particularly existed between Steed and Emma Peel, and the common question of "Will they ever go to bed together?" Clemens's attitude toward the characters was that they already had done, and this was the next day. Stars Patrick Macnee and Diana Rigg confirmed later, in interviews, that they had decided their characters had a casual sexual relationship, "but just didn't dwell on it."
Mrs. Peel's maiden name is Knight. We learn more about her childhood and her relationship with her mother and father in an exhibition on the "late Emma Peel" in the episode The Avengers: The House That Jack Built (1966).
Diana Rigg was the first person ever to do Kung Fu on the screen. In 1965, Ray Austin went to his producers and said, "Listen, I want to do this thing called Kung Fu". They said "Kung what?" and insisted that Emma, like her predecessor, stick to judo. Instead, Austin secretly taught Rigg Kung Fu.
A radio version began two years after the show ended: broadcast weeknightly on Springbok Radio, the South African Broadcasting Corporation's English-speaking wing, scripts from the filmed series (often earlier versions than had appeared on-screen) were reworked into fifteen-minute serials of varying lengths. Sponsored by Cold Water Omo and starring Donald Monat as Steed and Diane Appleby as Mrs Peel (Mother made occasional appearances, usually played by Colin Fish), the series ran from 6th December 1971 to 28th December 1973 (plus a mini reprise in "The Great Gong Robbery", a special drama celebrating Springbok's Silver Jubilee on 30th April 1975). Laurie Johnson's theme tune was used throughout, and to smooth over the more visual aspects Springbok news-broadcaster Hugh Rouse was engaged as the tongue-in-cheek narrator. This was South Africa's sole experience of the show (outside of rented film prints) at the time since their television service only began in the mid-'70s and the parent TV series wasn't purchased until many years later. It is unknown how many serials aired: from a potential 83 stories (some of which appear to have been remade), only 19 are currently known to exist in full, thanks to private South African enthusiasts as the SABC did not retain any copies.
When the series began, Ian Hendry was the main star, with the idea being he would rotate between different partners (an early version of the Mission: Impossible (1966) format). The series title actually refers to Hendry's character, Dr. David Keel, and Steed, who worked together to find those responsible for Keel's fiancée's murder in the first episode. Early episodes focused more on Keel's character, and Steed doesn't even appear in a couple! When the first season was interrupted by a strike, Hendry quit the series during the hiatus. The same format was used for Steed with a couple of leftover Dr. Keel scripts retooled for a new character named Dr. Martin King, but other scripts, originally written for a male character, were rejigged for another new addition: Catherine Gale.
Despite the reputation of "The Avengers", In Britain it only made the top ten of watched shows four times between 1960-69. February 1964, November 1965, March 1967 and February 1969. Ian Hendry's subsequent series The Informer (1966) was actually more popular at the time. So when the production company (ABC [Associated British Corp.], nothing to do with the American ABC, which stood for American Broadcasting Co.) was ordered to be wound up by July 1968 (due to breach of license on another matter), it was easy for the new company (Thames TV) to cancel the series.
A sequence dropped from Pulp Fiction (1994) had Vincent Vega confess to Mia Wallace about fantasizing over Emma Peel beating him up. Uma Thurman, who plays Wallace, later went on to play Emma Peel in The Avengers (1998).