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.
This is not the first time Hugh Bonneville (Robert) and Elizabeth McGovern (Cora) have played husband and wife. In fact, it's the third.
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.
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 basically, "No. You're rich, you have someone to iron your tails for you, you wouldn't think to do that."