First episode featuring Diana Rigg in the role of Mrs. Emma Peel that was filmed, completed December 14 1964.
Patrick Macnee reveals in the book 'The Avengers - The Inside Story' that the scene in involving the tuba was originally scripted with Steed playing the instrument. But feeling unprepared owing to the changes in the format at the start of the fourth series, Macnee insisted that Diana Rigg pretended to play the instrument instead.
Novello Award-winner Herbert Kretzmer provides the lyrics for the "Togetherness" theme. He had also co-written "Kinky Boots" with David Lee (associate of Venus Smith in the second season of the show), which became a (very slow-burning) tongue-in-cheek hit for Patrick Macnee and Honor Blackman in 1964/1990.
During a conversation with Lovejoy, Steed makes reference to being hanged if he ever murdered his fictitious cousin. On 9 November 1965, three days before screening, hanging had officially been abolished in England.
Steed agrees to killing Mrs Peel in exchange for someone else killing his fictitious cousin in order to inherit the family fortune. The concept of two people, unrelated, killing someone for the other is the main plot of Patricia Highsmith's 1950 novel "Strangers on a Train" and the 1951 film adaptation by Alfred Hitchcock.