* ActorSharedBackground: Hacker, Sir Humphrey, and Bernard share birth dates (and dates of death where applicable) with the actors who portrayed them.
* CastingGag:
** Michael Aldridge (who'd played the head of [=MI6=] in ''Series/TinkerTailorSoldierSpy'') as the head of [=MI5=]. Both stories involve the revelation that a senior member of British Intelligence was a Russian mole.
** Jim Hacker's previous role was Shadow Minister for Agriculture. In ''Series/TheGoodLife'' Eddington plays the neighbour of a man who has decided to grow all his own food in the garden.
* HarpoDoesSomethingFunny: Or in this case, as writer Jonathan Lynn reports putting in the margins of scripts, "Paul doesn't have to say this line if he doesn't want to". Paul Eddington, who played Hacker, had an amazing ability to convey the same sense a line was intended to give with an expression. One particularly good example is [[https://www.youtube.com/watch?v=I9NifqJyDMI 1:10 to 1:30 or so in this clip.]]
* ThrowItIn: The moment in "The Official Visit" where Bernard's briefcase falls open and lets his files spill out on the station platform was unscripted -- in fact, Derek Fowlds actually said "oh, shit!" when it happened, which somehow made it into the finished episode (probably because it was only just audible over the noise of the trains).
** Not surprising; Derek always said "shit" every time he goofed.
* WagTheDirector: Paul Eddington, a firm believer in nuclear disarmament, once convinced the writers to rework the script of a ''Yes, Prime Minister'' episode that he believed was rather too flippant about nuclear war.
* WrittenInInfirmity: In ''Yes, Prime Minister'' you might notice that in the later episodes especially, Jim Hacker spends much of the show sitting still behind his desk or in his armchair at home. This is because at the time of filming Paul Eddington was in the early stages of the T cell lymphoma that eventually killed him, and the episodes were written to be as comfortable for him to perform as possible.