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Actor-Shared Background: Mrs. Hughes wasn't originally written as Scottish, but that changed after Phyllis Logan auditioned.
Also, Branson was originally envisioned as being from Yorkshire, but after Allen Leech's audition, Julian Fellows realized making the character Irish opened up a whole new list of storytelling opportunities, and made Branson Irish instead.
Cora is an American who married an Englishman, moved to England with him, and spent the next several decades with him raising their daughters. The same is also true of her actress, Elizabeth McGovern.
Dawson Casting: The series starts out by doing this; for example, Lady Sybil is 16 at the time the show begins, but played by the 21-year-old Jessica Brown Findlay. Since the show's time progresses considerably faster than real time, this doesn't last; by the end of season 2, Sybil has actually passed Findlay's real age.
Dueling Shows: With the revival of Upstairs Downstairs. Downton came out the clear winner as an international mega-hit with three series completed and a fourth underway, while Upstairs was cancelled after its second series.
Fake American: Gary Carr is English — and unfortunately his rather stage-school attempt at a generic American accent makes this obvious.
Irony as She Is Cast: A slight variant: Rose Leslie, who played the housemaid Gwen who dreams of being a secretary, is possibly the single most aristocratic member of the cast, being the daughter of the chieftain of Clan Leslie of Aberdeenshire and the descendant of earls and Lords of Parliament—and of Charles II (through one of his many illegitimate children). Her full name is Rose Eleanor Arbuthnot-Leslie. She was literally raised in a castle, and her parents own another one. (It's especially stark when you consider that Michelle Dockery, who plays Lady Mary, is the daughter an Irish van driver-turned-surveyor and the great-granddaughter of, er, a housemaid.)
Sybil was killed off because Jessica Brown-Findlay wanted to leave to pursue her film career.
Matthew had an even more obvious case than Sybil's, because there had been buzz for months before his character's death about Dan Stevens possibly leaving the show. The writers have since admitted that they would have preferred not to kill off Matthew.
One of Us: In the Behind the Drama interview videos, Jessica Brown-Findlay and Allen Leech talk about scanning scripts of their scenes together and wondering when Sybil and Branson are finally going to kiss.
Attempted, but ultimately subverted. The original dog had to be replaced (he was male and got into a few fights with the dogs of Highclere's real-life owners.) They were going to initially merely replace him with a female dog, but that was shot down by Hugh Bonneville, who said that fans would be able to tell right away. So, on the spot, the story was changed to say that Pharaoh had died and was replaced by Isis.
Real Life Writes the Plot: Isobel Crawley went to France in series 2 because Penelope Wilton was busy starring in A Delicate Balance at the Almeida Theatre.
During series 3, Dan Stevens revealed that he wanted to leave the show, putting Fellowes in the quite awkward position of having to write Matthew out after he and Mary were married. Ultimately the only way out was to kill him.
Real-Life Relative: Kevin R. McNally, who played Mr Horace Bryant, is married to Phyllis Logan, who plays Mrs Hughes.
Shown Their Work: They've paid attention to some very small details about the Edwardian Era that might be overlooked. In one interview, Hugh Bonneville said that he was about to sit down in a suit and tails. He flipped the tails out behind him to avoid sitting on them and wrinkling them. Someone jumped in and said "No. You're rich, you have someone to iron your tails for you, you wouldn't think to do that."
Stunt Casting: The casting of Shirley Mac Laine as the Grande Dame Martha Levinson is done for all the right reasons — what other American actress could possibly go head-to-head with Dame Maggie Smith in the battle of the matriarchs?