I can think of more of these, but the ones that spring to mind are…
Franco Nero, who played “Django” in the 1966 Italian western of that name, turns up in Quentin Tarantino’s film Django Unchained asking Jamie Foxx (Django) what his name is.
The actual mayor of Portland, OR has a recurring role as the personal assistant to the mayor of Portland, OR in the TV show Portlandia.
Joanne Woodward, who famously played a woman suffering from multiple personality disorder in the film The Three Faces of Eve, 20 years later played a doctor treating a patient with MPD in the TV movie Sybil.
In Arsenic and Old Lace, one of the jokes was that Jonathan Brewster was a gangster on the run and he’d had plastic surgery to change his appearance. But his surgeon had been drinking and made Jonathan look exactly like Boris Karloff, which attracted unwanted attention.
In the Broadway play, they cast Boris Karloff in the role of Jonathan. Regrettably, Karloff wasn’t available for the movie version, so Raymond Massey played Jonathan in the movie.
This isn’t exactly what the OP is going for, but close: the producers for the 1970s sitcom “The Mary Tyler Moore” show had conceived of the Sue-Ann Niven character about a year before she actually appeared. The producers kept searching for the right actress to play the part, and kept repeating “We need a really chipper housewife type - y’know like Betty White! We have to find ourselves a Betty White type!” The casting people kept auditioning actresses and weren’t happy. “We need to get a real Betty White type,” they kept saying.
Finally, somebody said “Why don’t we ask Betty White to play the part?”
I know of a similar example with a musician instead of an actor. The 1998 film Velvet Goldmine is set during the glam rock era, and the soundtrack features a mix of actual recordings from the period and new covers/original songs. One of the bands in the movie is a fictionalized version of The Stooges, and the filmmakers assembled a pretty impressive group of younger musicians – including Mike Watt, Thurston Moore, and Mark Arm – to cover “TV Eye” and “Gimme Danger” for the soundtrack. But who would provide the Ron Asheton style riffs? How about…original Stooges guitarist Ron Asheton?
An easy one that amused me to no end was Creed Bratton, who played Creed Bratton on “The Office”. That the name on the fake ID that he used on the show, William Schneider, is his actual name made it doubly hilarious.
He did sort of a remake cameo on TV in That 70s Show where he played his father (a famous wrestler from the 70s).
They did the same thing when casting the Battlestar Galactica reboot. They were looking for a ‘Mary McDonnell-type’ to play the president, and finally wound up just casting her.
Danny Thomas did a cute cameo on That Girl!. He’s dressed as a priest and Marlo Thomas bumps into him at the end of an episode, so she can say, “Oh excuse me father” and he can reply, “That’s quite all right my child”.
I think there was something like this in* Bridget Jones’s Diary*. The British actor who played the guy she falls for at the end, Colin Firth, was actually in the book as himself (I think)…
Something similar happened during the making of Time Bandits, according to the IMDb trivia entry. Kevin goes back in time and sees Greek warrior in a helmet fighting a beast (Minotaur, maybe). After he is victorious, the script reads:
“The warrior took off his helmet, revealing someone that looks exactly like Sean Connery, or an actor of equal but cheaper stature.”
And when the movie got made, the role went to Sean Connery. Quite a coup, considering how early that was in Terry Gilliam’s directing career.
The movie All the President’s Men cast Frank Wills as a security guard who discovered the Watergate break-in. He was the IRL guard who actually found the burglars in the building in 1972.
In a different category perhaps, but interesting: in 2000, when That '70s Show needed to cast the role of Charo as she was in the 1970s they went and got… Charo.
I’ve never read (or seen) Bridget Jones’s Diary so I don’t know if there’s a reference to Colin Firth in the book, but I do know it was inspired by Pride and Prejudice and that the Colin Firth character isn’t just based on P&P’s Mr. Darcy but is actually named Mr. Darcy. About six years previously Firth had played Mr. Darcy in the BBC miniseries of P&P, and though there have been other P&P adaptations since then he’s probably still the living actor most associated with the role.
Hugh Grant had also previously appeared in an Austen adaptation (Sense & Sensibility), although I gather that in Bridget Jones he’s in the sleazy guy role which isn’t what he played in S&S.
The original concept of the TV show Bones included the idea that Temperance Brennan was also an author of mysteries starring detective Kathy Reichs. Actually, the show is based upon novels about Temperance Brennan written by Kathy Reichs. Reichs also appeared in an episode or two of the show.
The movie Alice’s Restaurant cast the real Officer Obie (William Obanhein) mentioned in the song as Officer Obie.
Geraldine Chaplin played her real-life grandmother in Chaplin.
Pelle Hvenegaard was named by his parents for a character in their favorite book, Pelle the Conqueror. He was cast in the lead when the movie was made.
In the movie 2010, Arthur C Clarke is sitting on a park bench feeding pigeons when Dr. Floyd meets with the ambassador. Later, there is a shot of the cover of Time magazine where the face of the US President is Clarke and the face of the Soviet Premier is Stanley Kubrick.