For one episode, "Do Not Forsake Me Oh My Darling", Number Six was portrayed by Nigel Stock due to McGoohan being away filming the movie Ice Station Zebra.

During the episode "Once Upon a Time", Number Six undergoes an intense form of brainwashing and interrogation in which his mind is reverted to that of a child and he is made to relive major events of his life.

Other episodes suggest that he was a spy or similar operative, though director Alex Cox stated in his 2017 book I Am (Not) A Number: Decoding The Prisoner that he was in fact a rocket engineer who resigned from his work because he felt his research was being misused.

[4] He is shown to be highly sagacious, if not a genius, with proficiency and expertise in subjects ranging from fencing, boxing and marksmanship to mathematics, languages, astronomy and craftsmanship.

McGoohan always denied the theory; in a 1966 interview in The Los Angeles Times, he stated that "John Drake of Secret Agent [as Danger Man was known in the US] is gone."

[7] Novels based on the series by Thomas Disch and David McDaniel also connect John Drake to Number Six, though these are generally not considered canonical.

In the opening moments of the 2009 mini-series, Michael, the amnesiac who would be known in that series as Six, discovers an old man in the middle of the desert being fired upon by security forces in some kind of an escape attempt.