Hiring of a big celebrity in a starring role (not a supporting or minor one), even if it's not the sole major role, in a movie, play, or TV show. Only, this celebrity isn't known for acting. In fact, this may be the celebrity's first acting job. A close cousin of Stunt Casting, only with a bit more risk, since the stunt castee doesn't have much experience (if any), and is practically carrying the project despite that inexperience.
So why is it done with such risks? Well the fact is that thissometimesworks (it tends to work better for stand-up comedians, singers and pro wrestlers, being close to what they normally do for a living anyway). Also even if the vehicle is a failure, the new actor may still have a career afterwards.
You are most likely to see I Am Not Leonard Nimoy play throughout the movie. If the work is about the celebrity in Real Life it overlaps with Autobiographical Role. If the celebrity's role is doing what they're famous for, it overlaps with Cast the Expert.
David Bowie in The Man Who Fell to Earth. Although he didn't really act before this, he studied mime and acting in The Sixties as his recording career slowly gained ground and incorporated his new skills into his stage act. He went on to appear in a number of films in both lead and supporting roles, such as Merry Christmas, Mr. Lawrence and Labyrinth, and performed on Broadway as the title character in The Elephant Man in 1980.
Neil Diamond in The Jazz Singer (1980). He won a Worst Actor Razzie for his performance, but this is a good example of a performer who managed to retain their original career after trying their hand at this trope; in fact, three of his biggest hits — "America", "Love on the Rocks", and "Hello Again" — were written for this film.
Terminator 2: Judgement Day almost did this for Billy Idol. He was originally picked to play the T-1000, but had to turn down the role because of a motorcycle accident. W.A.S.P. frontman Blackie Lawless was also considered for the role at one point but got turned down for being too tall.
"Weird Al" Yankovic in UHF, but it was not as much a leap for him as he was already accustomed to delivering comic performances and the movie was basically "what would TV be like if Weird Al did that instead of music parodies?"
Bette Midler in The Rose; though she had dabbled in the theatre in her early career, it was her first movie role. She went on to have a long and fairly successful career as an actress, reaching her peak in both fields at about the same time (the late 1980s and early 1990s).
Almost every lead character in Dream High has a Real Life career in a singing group. This is not surprising, though, as the premise of the Korean Drama is a bunch of kids at a high school which specializes in the entertainment business (like the 1980's show "Fame").
You Are Beautiful has several, including Lee Hong Ki from the group FT Island and Jung Yong Hwa from CD Blue. In a non-musical reversal, Jang Guen Suk started as a model before the show, and just released his first music album in April 2011.
Two big vehicles for Shaquille O'Neal: Kazaam and Steel. His first film, Blue Chips, had him in a supporting role (along with Penny Hardaway) in his natural element and was better-received.
Dwayne Johnson in The Mummy Returns and The Scorpion King partially counts - it was more of a 'breakthrough to non-wrestling fans' rather than a direct example of this trope. He's had a successful acting career since then, to the point that he's not even credited as "The Rock" anymore.
The 2010 The A-Team film cast former UFC champion Quinton "Rampage" Jackson as B.A. Baracus, filling the shoes of Mr. T, who was also known for his tough-guy exploits before getting into acting. Jackson had done a few films before, such as a memorable cameo in The Midnight Meat Train.
Rudolf Nureyev (ballet dancer) as Rudolph Valentino (silent movie star) in Valentino.
It was only a TV movie, but Vanna White of Wheel of Fortune fame starred in Goddess of Love on NBC. At that point in her career, the only other TV experience she had was… as a contestant on The Price Is Right in 1980 (two years before she took over the Wheel role she holds to this day).
Steven Soderbergh likes this trope, apparently stemming from his experience working with non-actors in the 2005 indie film Bubble.
Sodebergh did this for himself in his surreal 1996 comedy Shizopolis, which stars the man himself in not one but two roles. Underneath all its weird and surreal humor, it's really about Soderbergh examining the crumbling of his marriage (his real life ex-wife appears as his characters wife) as well as his anxieties about the future of his directing career in the wake of a series of flops following his debut sex, lies, and videotape
Nickelodeon also seems fond of this. Maybe not as fond of it as Disney, since most of the cast of iCarly and Victorious have acted before, one might say they invert it by taking moderately experienced actresses and using that to help launch their singing careers.