Nic Cage - Fast Times at Ridgemont High

Jimmy Buffett - Repo Man

I am disappointed to not see Kevin Costner in The Big Chill. Of course he was disappointed to not see himself in it too.
Kevin Costner was in The Big Chill as the dead body in beginning, but all of his flashback scenes were cut so that’s all you see of him.

Elijah Wood is in Back to the Future 2 as one of the children playing an arcade game at the throwback cafe

What happened to the post two weeks ago that was exactly this?

Jason Statham had his first speaking role in Collateral starring Tom Cruise. He plays a guy who bumps into Tom in the air port right at the beginning and they apologise to each other, and he plays no further part in the film. I think he may actually get the first line in the film but I'm not sure. Also he had hair then. (also he was an Olympic swimmer/diver before that)

Vin diesel in saving private ryan, small role but part of Tom Hanks squad

Orlando bloom was cast in the lord of the rings prior to fame, but became quite famous before lord of the rings release with black hawk down

Jeff Goldblum's first film role was an extra in Annie Hall starring Woody Allen and Diane Keaton. In a scene towards the end of the film, Allen's character is at a house party in the Hollywood hills, complaining to Keaton's character that everyone in LA is phony, braindead, and performatively spiritual. As he finishes his tirade, the film puts a button on his point by panning over to Jeff Goldblum. He's at this party, holding a phone to his hear and telling (presumably) his spiritual advisor, "I forgot my mantra!"

Such a dumb punchline, but it's lived rent-free in my head for decades now.

Zombies of the Stratosphere is a cheesy 1952 sci-fi movie starring a young Leonard Nimoy as a Martian henchman.

Not a movie, but a TV commercial.

Jack Black starred in an ad for Pitfall on the Atari 2600 when he was a kid.

2 more for Pitt. Thelma and Louise. True Romance.

Jonah Hill in I Heart Huckabees.

Clint Eastwood in Revenge of the Creature, which was viewed in season 8 of Mystery Science Theater.

Jack Nicholson has a small role as the masochistic dentist patient in the original Little Shop of Horrors. He's prominently featured on the DVD cover despite only having a small role because he's the only actor of that project who got famous.

It's awesome seeing him in a role where he doesn't play in his usual Jack Nicholson way.

Matt Berry in The Devil's Chair.

Just the other day we bought the DVD on a whim because it looked like a stupid movie and it was only 1€. We started watching with no expectations and were delighted when Matt appeared out of nowhere in a supporting role because we hadn't even bothered looking at the cast, expecting the usual complete no-names.

Movie was fucking atrocious btw and mostly not in a fun way, very much up its own arse, and Matt didn't even do the thing. Although he clearly chose his own wardrobe.

Not exactly what you asked for but Simu Liu modeled for stock photos before becoming famous.

David Dastmalchain. He was an extra for many years before his breakout small role in Nolan's second Batman movie as one of Joker's goons.

I even remember pausing the scene (because he is so good) on one viewing to look him up and saw it was his first role listed on IMDB. (He has since been credited for a few tiny roles prior.)

Now he's in a ton of movies and has Grave Conversations on YouTube where celebrities are interviewed in caskets.

Jackie Chan was a random henchman that Bruce Lee knocked out in Enter the Dragon.

Nick Offerman is in City of Angels, though you'd be forgiven for missing him without his magnificent 'tache.

Javier Bardem in Tacones Lejanos (high Heels) from Pedro Almodóvar. He's just a PA in a news program, and have just one line.

Cool thing to think about, I don’t know any though, ha, your list is cool.

(removed comment because oops, it was supposed to be an extra)