The bodyguard wearing the short, short shorts in the "Home" scene in A Clockwork Orange, plays Darth Vader in the the original Star Wars trilogy.

http://www.imdb.com/name/nm0001190/

Fear and Loathing in Las Vegas has a bunch of these. Off hand I remember:

Verne Troyer as the midget waiter.
Cameron Diaz as the reporter Del Toro threatens with a knife.
Ellen Barkin as the "Back Door Beauty?" waitress.
Mark Harmon as a reporter

Hot Fuzz had two 'holy shit that was...' moments after I looked at IMDB
Peter Jackson as the Crazy Santa
Cate Blanchett as Nick's ex-girlfriend.

I think the all time greatest example of not knowing a famous actor was in a film, was Gary Oldman in Hannibal as Mason Verger.

He remained uncredited, and people only figured it out, because there was a 'Assisstant to Mr. Oldman' in the credits.

That, and also every single review for the movie mentioned it was him.

One more. I was watching the 4th season of Married with children last night, and when watching the forgein exchange student episode, i kept on wondering who the really hot student was, since she looked really familar. Well turns out that its was Mila Jokovich. And wow, she was even hot back then (the episode was made in 89, almost 4 years before Dazed and Confused)

i never noticed that Alia Shawkat (Maeby Funke from Arrested Development) was the Iraqi girl who's mother is shot in Three Kings.

-NiCK

Watching Airplane! the other day I realized 27 years after the fact that Jimmie "J.J." Walker was the mechanic that falls off the nose of the plane.

The doctor in Crank is Dwight Yoakam and the desk clerk in Fear and Loathing in Las Vegas is Christopher Meloni from Law and Order

The other day, I realized 27 years after the fact that JJ Walker was the guy who just delivered my green pepper and onion pizza.

Another good movie to figure out who all the people are is Into the Night with Jeff Goldblum and Michelle Pfiffer. There are so many cameo's of big names that usually work behind the camera.

Also, the woman who asks Jack Nicholson's character "How he writes women so well?" is the same one that's on Showtime's Dexter and in the new Rambo movie.

Realized recently that Michael Cera played Gordo's son in Frequency.

That would have made her 14 at the time.

The FBI will be at your door shortly.

Oh shit. Better keep on the lookout for Chris Hansen.

I hear ya though. There's just something about her. She's not hot in the traditional way I guess, but she really does it for me.

Since it came out before The 40 Year Old Virgin and Knocked Up, I had no idea that the Veronica's cameraman was Seth Rogen and the guy who yells out "It smells like Big Foot's dick!" is Judd Apatow in Anchorman.

This is the inverse of the thread's subject, but I thought the guy who played Ed Miller in The Assassination of Jesse James... was Peter Krause of "Six Feet Under." It turns out it was some guy named Garret Dillahunt.

I swore that Heath Ledger in Lords of Dogtown was Jake Busey.

Although I'd seen it many times before, I was watching "What About Bob?" the other night, and had never put together that the daughter was played by Kathryn Erbe, who is one of the lead detectives on Law & Order: Criminal Intent.

Also, the prostitue who Bob pays to make the phone call for him is Aida Turturro, aka Janice from "The Sopranos".

Mandy Patimkin on "Criminal Minds." I had seen the Princess Bride a few times many years ago, had NO idea he was... Inigo Montoya!

Jeremy Piven as a sailor on the train in The Grifters

Doesn't a woman say "it smells like Big Foot's dick"?

Dustin Hoffman as Captain Hook in Hook. I knew that subliminally, but after seeing the Graduate and doing some study work on him, it just made me acknowledge the fact. He's deceiving because of how tall they make him seem!

I saw Hoffman once at the Century City Shopping Center (same day I saw Julia Roberts there), he is a petite man...

Here's how I found out:

A) I was reading Dark Horizons the day they announced Gary was playing Mason Verger.

B) I was reading Dark Horizons the day they announced Gary didn't want to be credited for playing Mason Verger.

C) In the flashback scene, you can see that Gary Oldman is playing Mason Verger (even when he