Ascended Fanon: When a series 2 sketch featured a very Camp friend of George IV's, fans assumed him to be BeauBrummel. When the character reappeared in series 5, he was explicitly identified as Beau Brummel.
Conversely, the un-named young general who has his hair sniffed by Alexander the Great in the infamous first-series "Alexandria" sketch was assumed to be Alex's best friend (and possibly lover) Hephaestion. When Alex got his solo song in series 5, however, Hephaestion was explicitly indicated to be another actor in a very different makeup, subverting this trope completely.
The Danza: The Norman dinner party segment is based on Abigail's Party, allowing Laurence Rickard to play Laurence.
Descended Creator: Rickard was originally hired as a writer but was summarily promoted from back of the camera to front in the first series after creating Bob Hale, 'News at When' special correspondent, and his extended monologues. Rickard then took on other small supporting roles, proved versatile and popular, and by the second series was established as both a senior writer and part of the starring troupe.
Fake Nationality: All the very English cast are also fairly adept at sounding Scottish, Welsh, Irish, French, etc. Baynton even gamely tackles a Californian accent-as a Steve Jobs Expy-for the 'aBook' sketch (that said, their ideas re: American 'cowboy' drawls are a bit less impressive).
One World War One skit calls for French Canadian, Australian and South African accents. It doesn't really work, but major points for effort.
Hey, It's That Guy!: Gerard (Jim Howick) from Peep Show, now the proud owner of a children's BAFTA for Best Performer; besides Deano (Mathew Baynton) from Gavin and Stacey and-in perhaps one of the most hilariously dissonant examples in modern TV-Adam Kenyon (Ben Willbond) from The Thick of It as well. All of which means this trope can become distinctly awkward when applied in reverse.
Queen Victoria is the other major example. She was played by Sarah Hadland in old-age makeup (the sketches usually focusing on her mourning Prince Albert) in series one, but afterwards she was played by Martha Howe-Douglas from age 18 (her coronation) to her mid-80s.
Type Casting: Both played straight and occasionally subverted by the producers when casting sketches. Young and naive types were usually played by Baynton; the craziest characters usually went to Farnaby; Magnificent Bastards to Willbond; the (very) average schlubs to Rickard; and the cute little Woobies, frequently with jerkass overtones, to Howick.
Written By Cast Member: Performers Baynton and Willbond received several writing credits in Series Two (including Baynton's co-writing credit for "Literally (The Viking Song)".) In addition, improvisation among the starring cast was encouraged, and even sometimes necessary thanks to the rapid-fire pace of filming.