The character "Bowler" is portrayed by an actor named John Drake. Not only is John Drake the name of Patrick McGoohan's character in Danger Man (1960), but is also possibly the true identity of No.6 in this show. McGoohan hired Drake because he was amused by the coincidence.
"The Girl Who Was Death" was originally conceived as a two-part story. A psychedelic piece with the hero in a maze was one of the sequences that never got filmed. This was envisaged for the movie or two part version. Lew Grade said a long version would never sell.
The Sherlock Holmes disguise in the boxing match and the funfair was not in the original script. It was changed to allow Frank Maher, Patrick McGoohan's stunt double, to play No 6 during McGoohan's absence from filming.
This episode contains three actors from the "Danger Man" film Koroshi (1968) - Kenneth Griffith, Patrick McGoohan, and Christopher Benjamin, who appears as a character called "Potter" in both. (Another "Koroshi" actor George Coulouris also appears in "Checkmate"). While Benjamin claims he was unaware of the connection, and played them as different characters, some fans claim that they are connected, and this is another link which proves that No 6 is in fact John Drake from Danger Man (1960), something which McGoohan denied, but which George Markstein (who was involved in the earliest stages of development) affirmed. "The Girl who was Death" in fact started life as an unproduced script of "Danger Man".
Patrick McGoohan was working on the film Ice Station Zebra (1968) at the same time as "The Girl who was Death". He had already shot some scenes for the film, but had to return to Hollywood to complete his part, and was absent from England for nearly six weeks, so The Prisoner (1967) crew had to shoot round him, and use a double for some of the scenes.