I find myself saying that alot recently while watching older movies in my collection that i havent viewed and a year or two. Some recent occurences was

Resident Evil-Guy who plays Matt is the boss on the tv show Ugly Betty
Donnie Darko-Seth Rogen plays as the bullies friend
Wet Hot American Summer-First time i watched it i only recognized a few people, went back and recognized a bunch more including Paul Rudd

What movies have you went back and watched and realized that someone who you either know now or that has become bigger since they stared in that movie

Just watching Collateral with Tom Cruise.

Never caught that Jason Statham was the guy he bumped into at the airport at the beginning. Cool stuff.

This maybe related:
http://forum.dvdtalk.com/showthread.php?t=431083

I just re-watched The Rocketeer (1991) yesterday for the first time in many years. Although I remembered from before that the guy who played Howard Hughes in it did a great job in the role, I didn't remember which actor played him. So I was very surprised to clearly recognize Terry O'Quinn (aka John Locke from "LOST" and Peter Watts from "Millennium") as the actor playing Howard Hughes (although that at least explains why I still remembered the role being played so well because he's good in everything).

jack black from AIRBORNE (1993)

William H. Macy with a very tiny part (a TV production supervisor or something) in The Last Dragon, starring Taimak.

Jack Black in Waterworld. I also remember laughing when I saw Tim Robbins as Merlin in Top Gun

Paul Ruebens in the Cheech And Chong movies.

I never realized that Claude Rains played Senator Payne in Mr. Smith Goes to Washington until I looked the movie up on IMDB one time. I've seen Rains in Casablanca/Notorious, and he looked completely unrecognizable in Mr. Smith.

Jack Black was also the super-military brother in "Mars Attacks!"

Jack Black was also Jim Carrey's best friend in Cable Guy.

Katherine Heigl was the girl in "Under Seige 2ark Territory"!

Another in The Rocketeer - I was wondering who played the hot nightclub singer in a section of the movie, so I looked it up and discovered it was none other than Melora Hardin (aka Jan from "The Office").

Dominic Purcell of Prison Break fame as a random thug in Mission Impossible 2. He has even less lines than Cruise's leech of a cousin.

Jack Black was also Sean Penn's brother in Dead Man Walking.

Jack Black was also one of the crazy fans in Bob Roberts

Earlier today I was watching a DVD my girlfriend from Canada sent me from the Trailer Park Boys box set, and saw Ellen Page (Kitty Pryde from X-Men 3) as the trailer park supervisor's daughter.

Super Dave in Ocean's 13.

This thread should be merged with the one above.

Jack Black wins this thread so far.

Surprised to find Beth Grant in quite a few movies after seeing Donnie Darko, including Speed, Child's Play 2 and a momentary appearance in The Wizard.

Also amused that Reginald VelJohnson is a cop in Ghostbusters. Perhaps the same one from Die Hard?

Jack Black wins this thread so far.

Surprised to see Beth Grant in many things after watching Donnie Darko, notably Speed, Child's Play 2 and a blink-and-you'll-miss-it bit in The Wizard.

Also amused that Reginald VelJohnson is a cop in Ghostbusters. Maybe Al worked in NYC too?

Jack Black was also in The Fan and Waterworld. Personally i have been surprised at some of the movies that Billy Bob Thornton did. In the last week i have noticed him in "On Deadly Ground" and Tombstone.

Jack Black was also in Neverending Story 3..... i cant believe i watched that movie as well..

Mario Lopez was in Colors as one of the random gang members

"President" Dennis Haysbert in Major League (and its sequels) as Pedro Ceranno

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Here are the ones that I always forget and then am surprised to see that they were in these films:

1. Paul Reubens in Blues Brothers
2. Jim Carrey/Liam Neeson in The Dead Pool
3. Kevin Bacon in Animal House
4. Jessica Alba in Never Been Kissed
5. Orlando Bloom in Black Hawk Down
6. Jack Black in Demolition Man
7. Alfred Molina in Raiders of the Lost Ark
8. Britney Murphy in Drive
9. Samuel L. Jackson in Goodfellas
10. Keanu Reeves in Parenthood
11. Dennis Haysbert in Heat
12. Michael Wincott in Basquiat
13. Damon Wayans in Bev. Hills Cop
14. James Gandolfini in any 90s movie (pre-Sopranos)
15. Anthony Keidis in Point Break
16. Actors in Vietnam movies (Platoon, Born on 4th, Apoc. Now) - everytime I watch a Vietnam movie, there is always someone with a tiny role who eventually becomes famous.