Jim Carey made a secret second appearance in Liar Liar, appearing as his Fire Marshal Bill character in the background of one scene.
In Back to the Future Huey Lewis, (yes, the one who wrote The Power of Love) plays one of the stuffy judges from the beginning of the film when Marty is auditioning.
Conan O’Brien won an auction to appear as an extra on How I Met Your Mother. When the producers found out they wanted to give him a bigger part, but he refused to be more than a background character.
George Lucas appeared in Beverly Hills Cop III as a park goer, though he is officially credited as “Disappointed Man.”
Cate Blanchett is hard to recognize behind her surgical mask and full body suit, but it is her playing Nicholas Angel’s Ex-Girlfriend in Hot Fuzz.
Blink and you’ll miss it, but Glenn Close dawns a beard and a wig to play this pirate in Hook who is stuffed into a box.
Iggy Pop appeared in Star trek: Deep Space 9 in a 1998 episode, “The Magnificent Ferengi.”
Ron Jeremy can be spotted in the crowd in one scene in Ghostbusters.
Bruce Campbell appears in the Cohen Brothers movie Fargo. Though he didn’t act for the role, they used old footage from a soap opera he was on.
Stephen Colbert boasted about being in The Desolation of Smaug and few believed him, but he was hidden behind makeup and an eye patch amongst the folk of Lake Town.
Director Tim Burton, for some reason, decided to play a corpse in Hoffa. He has no other affiliation with the film.
Dan Aykroyd only appears for one scene in the Temple of Doom, for a total of 15 seconds to boot. He returned the favor to Spielberg for playing the role of an office clerk in Blues Brothers.
There were a lot of movies starring Detective Harry Callahan, better known as Dirty Harry. In the fifth, The Dead Pool, Guns N Roses appear as funeral goers.
Flea from the Red Hot Chili Peppers plays one of the Nihilists in The Big Lebowski.
Rage Against the Machine frontman Tom Morello worked on Iron Man as a consultant. They gave him a small spot in the film as one of the terrorists that tony Stark escapes from.
Justin Timberlake takes front stage in the “This Train Don’t Stop There Anymore” music video, but Paul Reubens (A.K.A. Pee Wee Herman) can be spotted playing his manager.
Brad Pitt and Matt Damon went uncredited as two bachelors on a dating show who were not chosen in Confessions of a Dangerous Mind
The story goes that George Clooney was a big fan of South Park and asked to be given a guest spot. So they let him voice Stan’s gay dog Sparky.
In a similar story Jerry Seinfeld asked the same and was offered the role of a turkey for a thanksgiving special, and he was insulted and did not accept the role.
Sigur Rós, a rock band from Iceland took a turn in Game of Thrones when they played the wedding of Joffery and Margerie.
They were not well received.
Will Champion, the drummer for coldplay also got a musical spot in Game of Thrones when he played another wedding. The Red Wedding.