Mark Gatiss stood in for Matt Smith during a scene in 'Doctor Who' origin drama 'An Adventure in Space and Time'.
The 37-year-old actor - who played the Eleventh Doctor - was meant to make a cameo in the 2013 programme to mark the 50th anniversary of the sci-fi show, but he was unable to make it to filming, meaning screenwriter Gatiss took his place.
In a watchalong of the special, the 53-year-old star tweeted: "Right up till the last moment we were hoping Matt could actually shoot his cameo onset but scheduling was impossible. So those are my hands!"
Brian Cox played the role of series co-creator Sydney Newman and Gatiss admitted that he was determined to cast the 'Succession' star after seeing him on 'The One Show'.
He wrote: "Sydney was such a colourful character he was very hard to cast. We discussed John Lithgow and even James Gandolfini.
"Then I saw Brian Cox on 'The One Show' and - pop! pop! pop! Brian even changed his holiday plans so he could do it."
During another recent watchalong, former 'Doctor Who' showrunner Steven Moffat joked that he left a continuity error in 2014's 'Listen' as he was "drunk with power".
The episode starred Peter Capaldi as the Twelfth Doctor and Jenna Coleman as companion Clara Oswald, and Steven opened up about a mistake in his script - a reference to William Hartnell's Time Lord from the 1960s.
He tweeted: "'Fear make companions of all of us!' The very first time the Dr is nice to anyone in Dr Who, those are his words (we had to wait till ep 3.)
"Always loved the line, and the kindness that first emerged with it. This was my love letter to it. And in fact, I misquoted the line.
"Hartnell says 'of all of us' and Jenna says 'of us all.' Knew it was wrong, but couldn't let go of the way I'd misremembered it for so long.
"Like, was the DG gonna fine me cos I misquoted Hartnell. Drunk with power, I was."