1. Robert Holmes was approached to write the 20th Anniversary special, he came up with a concept called "The Six Doctors". In his treatment the Cybermen kidnapped the five incarnations of the Doctor in an attempt to extract Time Lord DNA and turn themselves into "Cyberlords". The reason it was called The Six Doctors was that it would turn out that the First Doctor (and Susan) was actually an android impostors - the sixth Doctor. Robert Holmes withdrew from the production early on, and Terrance Dicks ended up writing the adventure we know, but Holmes did go on to use some of the ideas he had for this story later in The Two Doctors.
2. Mind you, it very nearly was The Six Doctors. Colin Baker had already appeared as Security Commander Maxil in the opening story of Season 20, Arc of Infinity, and the original script saw him and Elspet Gra's Chancellor Thalia return for the anniversary special. Work commitments kept them both from reprising their roles, and so their parts were given to Stuart Blake (as an anonymous Commander) and Dinah Sheridan (as Chancellor Flavia).
3. The shows producer at the time, John Nathan-Turner, originally invited Doctor Who's very first director, the man behind the camera for An Unearthly Child, Waris Hussein to return to direct the special, but he declined as he had recently moved to America where he spent most of the 1980s directing made for TV movies. JN-T then asked another veteran Doctor Who director Douglas Camfield (who had actually been responsible for directing some of the film inserts used during An Unearthly Child, but remained uncredited), he also declined due to ill health.
4. As William Hartnell had passed away in 1975 a 'new' First Doctor would need to be cast. One of the names considered early on was Geoffrey Bayldon. You may know his as Catweazle, or as the Crowman in the Jon Pertwee serial Worzel Gummidge, but you might not know that he was actually offered the role of the Doctor back in 1963 before William Hartnell. He declined back then, and in the end was never approached in 1983 as one of the production associates suggested Richard Hurndall after remembering him from an episode of Blake's 7 and thinking he looked a lot like Hartnell.
5. Terrance Dicks original script included an appearance by the Autons, who had not been seen for over a decade since the 1971 story Terror of the Autons. They would've appeared in a scene set in an abandoned Gallifrey town, the area of the Death Zone where Sarah Jane Smith was to appear. She would then be saved by the Doctor. But due to budgetary restrictions this was dropped and instead Sarah Jane is rescued from the 'gradual incline of terror'!
6. You may remember that scene in the story playing out with Sarah Jane and the Third Doctor, but it was supposed to be the one who was "all teeth and curls" rescuing her. The pairing up of Doctors and companions was an ever changing headache for the production team. Originally the Second was to be with Jamie, the Third with the Brigadier and the Fourth with Sarah Jane. Frazer Hines had to withdraw due to other commitments, so he was initially replace by Deborah Watling returning as Victoria Waterfield. Then Tom Baker decided he didn't want to return to the role so soon after leaving (his Doctor and Romana eventually appearing in un-transmitted footage from Shada) so everyone was switched around to the pairings we saw. Watling's part was drastically reduced, and she was set to appear as a phantom alongside Wendy Padbury as Zoe. Watling was cast in another production and reluctantly withdrew, so at the eleventh hour Frazer Hines cleared his schedule for a single day to film the small cameo.
7. Other companions who were approached about appearing in The Five Doctors include; Ian Marter who had been contacted about reprising his role as Harry Sullivan, but he had commitments in New Zealand so had to decline. Katy Manning was approached about reprising her role as Jo Grant, but the budget couldn't afford to fly her to the UK from Australia. John Levene was asked to play Sgt Benton which he initially was interested in, but objected to the script requiring Benton to not recognise the Second Doctor when he arrived at the Brigadier's office. Feeling this was unfaithful to his character, he declined to participate. Louise Jameson was approached to see if she'd be interested in returning to the role of Leela, she was and she offered her services. Unfortunately Terrance Dicks felt there was no opportunity to insert her into the storyline.
8. The part of Rassilon was originally offered to Charles Gray who, amongst other things, had played Ernst Stavro Blofeld in the James Bond movie Diamonds Are Forever.
9. In 2013 Carole Ann Ford revealed that when she had first received her script she'd discovered that every time Susan was addressing her Grandfather she would call him "Doctor". After taking this up with the producers, and pointing out that this was not true to her character, she revealed:
"You will not believe why. They said, 'We don't really want people to perceive him as having had sex with someone, to father a child.' I just screamed with hysterical laughter and said, 'In that case, I'm not doing it.'"The script was changed and she quite rightly called him "Grandfather".
10. The Five Doctors was broadcast in the UK on November 25th 1983 as part of that years Children In Need celebrations. But it had already premiered two days earlier in the USA on Doctor Who's actual anniversary. This means that The Five Doctors was the very first Doctor Who story to not receive its premiere broadcast on UK television.
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Also
10 Things You Might Not Know About The Daleks (1963 Serial)
10 Things You Might Not Know About The Three Doctors
10 Things You Might Not Know About Doctor Who: The TV Movie
20 Things You Might Not Know About Doctor Who: Series One
