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 Fellowes realized making the character Irish opened up a whole new list of storytelling opportunities, and made Branson Irish instead.
Thomas being gay was inspired by an ad-lib that Rob James Collier did during a table read. Fellowes was inspired to make it a character trait, and keep Thomas around past Season 1.
Cora is an American who married an Englishman, moved to England with him, and spent the next several decades with him there raising their daughters. The same is also true of her actress, Elizabeth McGovern.
Michelle Dockery, Laura Carmichael, and Jessica Brown Findlay all grew up in households where they had multiple siblings who were all girls. Laura Carmichael was even the middle child in a family with three daughters.
Colbert Bump: Saturday Night Live's surprisingly accurate summary of the show attracted a lot of attention to it: "There's a MILF and a dad and they've got three daughters named Hot (Mary), Way Hot (Sybil), and The Other One (Edith). And they all hang out with this old lady that looks like a chicken. We hated her at first, but then we got high and she made us crack up![...] There's also a whole bunch of tuxedo people who live in the basement and their lives suck! Get this: They always have to stand up at the same time. Their names are: Nice Guy (Bates), Mean Guy (Thomas), Mouse Girl (Daisy), and Super Bitch (O'Brien)."
Maggie Smith has been quite vocal on how glad she was when the show ended as she finds its immense popularity annoying since she can no longer go around in public without being approached by fans.
Elizabeth McGovern revealed shortly after filming finished that she was quite dissatisfied with the show for most of its run, feeling like she was "strangled" as Cora had hardly anything to do.
Everyone in the cast hated shooting the dinner scenes, which often took far longer to film than they should have, with the food quickly starting to stink.
Dawson Casting: The series starts out by doing this; for example, Lady Sybil is 17 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. Maggie Smith likes to joke that she imagines Violet was somewhere around 110 by the time the show ended.
Divorced Installment: The show was originally thought up as a spin-off to Gosford Park, also written by Fellowes. It later became a separate entity set a couple of decades earlier.
Dueling Shows: With the revival of Upstairs Downstairs (notably, the announcement that Shirley MacLaine had been cast as Cora's mother was deliberately timed to upstage the press launch for Upstairs' second series). Downton came out the clear winner as an international mega-hit for all six series, 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.
Hostility on the Set: On the later seasons, Julian Fellowes' tendency to do rewrites on set, with most of the actors having to remain in their tight Edwardian costumes under hot lights as spoiling dinner food fouled the air, did not endear him to many of the main cast. Others had also been alienated by his vindictive tendency to have minor characters killed off offscreen when the actors were too busy doing other work to come back to the show. By the end of the series many were swearing they'd never work with him again, although some did for the 2019 movie.
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.
Rule 34 Creator Reactions: The cast had mixed reactions to the porn parody Down on Abby. While being interviewed on Conan, Michelle Dockery remarked of her porn doppleganger "That's me, is it? The one with the extremely large breasts?". During a red carpet event, Hugh Bonneville said he thought the title was hilarious and that if somebody got him a copy, he would invite the rest of the cast to watch it.
Those Two Actors: This show marks the third time in their careers that Elizabeth McGovern and Hugh Bonneville have not only starred in the same production, but played husband and wife.
Troubled Production: The first season went pretty swimmingly, ending up as one of the most acclaimed shows of the year...and then spent the next five years in a morass of problems.
Many of the actors grew increasingly unhappy with how little they were given to do as the show's scope grew larger, with several of them quitting along the way. Some of these were major characters, forcing Julian Fellowes to hastily write their deaths to explain their absence. The most damaging was Dan Stevens as half of the show's major romantic couple Matthew Crawley. Fellowes was so furious at Stevens for this that he refused to so much as let his name be spoken on set for the remainder of the show, and even downplayed Matthew's huge role in the first three series as much as possible, with him hardly ever being mentioned again. This led to the show feeling more like a trashy soap opera and less the highbrow drama as intended, and reviews quickly soured.
Fellowes' obsession with Scenery Porn and portraying the lives of the early 20th century upper class led to more and more discomfort on set, especially his insistence on writing scenes around the dinner table which he then micromanaged and caused to take far longer to film than they should have...all while the food went bad under the lights and everyone had to suffer the stink.
Finally, his writing strategy was to only write the first half of each series ahead of time, then wait to write the rest until filming was underway and he could see how the actors played off each other. This sometimes resulted in some very awkward storytelling as he decided to beef up a character's role, only to discover their actor couldn't commit to the show any further. The worst of these was Charles Edwards as Edith's love interest Michael Gregson, whose departure was so clearly a bad story option that Fellowes simply had him go missing in Germany and left his fate unknown for two years...before finally giving up and having Gregson discovered to have been killed.
By the time the show ended, it was widely regarded as a joke and Fellowes' slow Creator Breakdown over all the problems left his reputation quite a bit lower than when he'd started.
A wild rumour circulated that Season 6 would end with the Crawleys leaving Downton Abbey and settling into Downton Place. The scenes that suggested this later turned out to be the Crawleys attending an auction at another house.
Fellowes has stated that even if the elderly Maggie Smith ever became unable to continue with the show for any reason, he would not have killed Violet off and would have come up with some reason for her not appearing onscreen, likely going on a long trip. Luckily, she remained with the show for its entire run.
Allen Leech has revealed that Matthew was originally going to end up with Sybil, until both actors tired of the show.
Writing by the Seat of Your Pants: Julian Fellowes only wrote the first half of each series ahead of time, then waited to see how the actors played off each other before writing the second half. Occasionally this bit him in the ass when he decided to expand a character's role only to discover the actor wasn't available, most notably with Edith's paramour Gregson.
A final scene to Thomas's storyline, leaving his budding relationship with Ellis on a more ambiguous and bittersweet note. It was ultimately cut to end the storyline in a more conclusive way — given the high possibility that the movie is the last thing we'll see from the Downton cast — and because Julian Fellowes, Robert James-Collier and others all agreed that Thomas deserved a happy ending.