And Professor Sprout as an Infanta of Spain in series 1, Blackadder's aunt in series 2 and Queen Victoria in Blackadder's Christmas Carol.
Three of The Young Ones have also guest starred on the show - Rik (Rik Mayall) as Mad Gerald in series 1 and Flashheart in series 2 and 4, Neil (Nigel Planer) as Smedley, the Scarlet Pimpernel's associate in series 3, and Vyvyan (Adrian Edmondson) as the Red Baron in series 4.
Arnold Rimmer as the French prison guard in Blackadder the Third.
Hagrid plays Dr. Samuel Johnson, inventor of the Dictionary, in series 3. He's also the ghost of Christmas in Blackadder's Christmas Carol.
Arthur Dent is Sir Walter Raleigh in "Potato". So despite their both being friends of Douglas Adams, this is the first and only time Arthur Dent and the Fourth Doctor have ever met on-screen - without his involvement!
Ford Prefect is the Standing at the Back Dressed Stupidly and Looking Stupid candidate as well as the Speaker of the House of Commons (the latter is only heard near the end) in "Dish and Dishonesty".
Word of God: Turnips are mentioned a lot, from the first series on. In series three, they become Baldrick's only ambition in life. The creators say this is because someone confused them with parsnips, which explains the joke about their shape in series two.
Present in The Black Adder
Old Shame: Rowan Atkinson and producer John Lloyd spend a significant amount of time badmouthing this series in the documentary included on the DVD boxset, calling it "pretty, but dreary to watch."
The Pete Best: BRIAN BLESSED (Richard), Elspet Gray (Gertrude), Robert East (Henry) all qualify, as none of them would return for the later, more popular incarnations of the series (Blessed was asked back for one-off appearances in the third and fourth series, but was unavailable on both occasions).
What Could Have Been: The original pilot episode was much more similar what the later Blackadder installments would be like, but Executive Meddling resulted in the version of The Black Adder that we know and... well, treat as a valued and respected friend, but not really love, per se. Most fans believe that had the finished series combined the pilot episode's characterisations with the cast that we ended up with, it would have been vastly superior.
In The Archbishop, Wilfrid Brambell was originally cast to play the dying landowner, the Duke of Winchester in the opening scene. However, delays while filming the scene led to Brambell getting impatient and storming off the set, resulting in his speedy replacement by William Russell.
Present in Blackadder II
Irony as She Is Cast: Miriam Margolyes, who played the Puritanical Lady Whiteadder, is not only openly gay but Jewish.
Name's the Same: Mr Ploppy and Mistress Ploppy are not married or related in any way, but have the same name by pure coincidence. Baldrick then offers to change his name to Ploppy "if it'll make things easier" on Edmund. For the rest of the episode, Mr. Ploppy refers to Baldrick as "Young Ploppy".
And of course, Ploppy's father Daddy Ploppy, AKA "Ploppy the Slopper".
What Could Have Been: BRIAN BLESSED claims that the original plan was that he would have played Queenie. And been madly in love with Edmund. It's quite a terrifying idea.
Present in "Blackadder the Third"
Present in Blackadder Goes Forth
Actor Allusion: In "Captain Cook", Melchett tells George that his Uncle Bertie sends his regards. Blackadder predates Jeeves and Wooster, so it's a prescient Actor Allusion!
Troubled Production / Wag the Director: According to Stephen Fry and Tony Robinson, there was a lot of friction between the cast and writers during the making of this series, with the actors frequently rewriting the script on the set, which the two have admitted doing themselves. The resulting atmosphere on-set — together with producer John Lloyd leaving the BBC and the knowledge that any hypothetical Blackadder 5 would be eviscerated by the critics if it was even slightly worse than the previous series — made it certain that this would be the final Blackadder production until Back & Forth ten years later.