Neil Patrick Harris had no idea what Doctor Who was when he was given a role on the programme.
The How I Met Your Mother star, 50, will appear as a guest role in the forthcoming special The Giggle, the third of three episodes featuring the return of David Tennant and Donna Noble as the Doctor and his companion.
While little is known about the last special, Neil portrays the Toymaker, a threat from the Doctor’s distant past.
However, creator Russell T Davies has said that while he knew he wanted the actor for the role, the American was completely unaware of the long-running series, before starring in the 60th anniversary special.
‘He’d never heard of it in his life, bless him,’ he added, referring to Neil’s lack of familiarity with the popular science fiction series when asked for a leading part.
‘I was fortunate enough to work with the wonderful guy on a production called It’s a Sin, which was about the 1980s AIDS issue, and working with him was such a delight.
‘The Toymaker, he’s kind of the god of games, so he shuffles cards, he does magic tricks, and all of that fits Neil Patrick Harris,’ he told EW.
‘If you go through agents, they often tell you to go away. I was able to send just a text saying, “Do you fancy reading this?” He read it and literally phoned me up going, “Let me get this right, so the Doctor’s an alien, right?”.’
Clearly taken aback Russell recalled: ‘I was like, “Oh my god, you really have never heard of Doctor Who!”. But he couldn’t resist it, and he came to Cardiff, and we had the most spectacular time.’
The Toymaker initially featured on the show in the 1960s, when Michael Gough played the role and William Hartnell played the Doctor.
Russell also told the newspaper that it was “very difficult to find the villain who can match” David and Catherine.
‘To have a character who can be in danger of defeating those two is very hard to find. Sometimes on Doctor Who, you need armies of a thousand robots who could do that.
‘This is just one person, so casting them was absolutely crucial and this becomes a pivotal event in the Doctor’s life. We needed that man, and, god, we had a glorious time.’
UNIT chief Kate Lethbridge Stewart (Jemma Redgrave), who will have her own spin-off, Charlie De Melo (Coronation Street) as Charles Banerjee, and Alexander Devrient (Ted Lasso) as Colonel Ibrahim will also feature in the episode.
It will also star John Mackay (The Hollow Crown) as John Logie Baird, a historical person who pioneered an early version of television.
The final Doctor Who special, The Giggle, airs on Saturday at 6.30pm on BBC One.
