Elijah Wood in Back to the Future 2 as that little brat on the arcade cabinet.

“We ain’t found shit!” Tim Russ as an unnamed Spaceball before becoming Lt. Commander Tuvok on Star Trek Voyager.

Samuel L. Jackson is the highest grossing actor of all time. His first non-independent film role was Gang Member No. 2 in the 1981 film, Ragtime.

My boy Adam Scott (main guy in Severance) in Star Trek VIII: First Contact as Worf's helmsman.

Here's his scene: https://www.youtube.com/watch?v=o-KoydNOWGc

Would Jack Black in Waterworld count?

Robert Downey Jr in Weird Science.

Not exactly the same, but Harrison Ford was working as a carpenter on Star Wars and was asked to just read in some lines as Han Solo while they were casting the other roles. Apparently he did a good job.

He had already acted in American Graffiti so I guess that's why they asked him to read in.

Marisa Tomei is in the background during a scene in The Toxic Avenger (1984). No lines, just screams and runs.

Matthew McConaughey in an old Unsolved Mysteries episode.

"freak #1". That's some chaos theory right there.

Jackie Chan is an unnamed martial arts student in Fist of Fury and a random mook who gets killed by Bruce Lee in Enter the Dragon. He mostly worked as a stunt double at the time, and was so good at stunt work he transitioned into credited performance.

Peter Capaldi as Caecilius in Doctor Who (The fires of Pompeii) before joining Doctor Who as The Doctor

Bill Paxton in Terminator.

Liam Neeson and Robbie Coltrane in Krull (1983)

John Cusack in '16 candles': he was one of the nerd friends who takes pictures of the unconscious cheerleader. And Brad Pitt in 'True romance' he was the roommate who needed cleaning supplies.

Starship Troopers. Casper Van Dien was an background extra in some of the episodes of Saved by the Bell.

Jean Claude Van Damme dancing in a skin tight onesie in the background in Breakin’ (1984). A breakdance movie

https://m.youtube.com/watch?v=bt6-F11LZsQ&ra=m

Jennifer Lawrence had one line in an episode of Monk

Brent Spiner on the gameshow To Tell the Truth from 1972. I used to watch it but I have no memory of that episode.

https://youtu.be/r6f4EK9fvIg

Jerry Doyle from Babylon 5 was an extra on Moonlighting (Bruce Willis). The scene was Jerry Doyle was in a line of actors for a casting call to replace Bruce Willis because Bruce Willis was threatening to quit.

chris pratt in the movie "wanted"

Sam Rockwell is a punk in the original Teenage Mutant Ninja Turtles.

Side note: Secret of the Ooze is a very fun movie, and I find it worthwhile to this day!

not exactly this but I have been watching old sitcoms and finding it funny where I see people. jaws from james bond was on I dream of jeanie. alice from alice was on barney miller and maybe twice as both a cast member and a one episode character. they guy who owns the radioa station from news radio was on murphy brown. elaines boyfriend who also played the tick played a reoocurring role on news radio.

I remember seeing some movie (don't remember the name) with a very familiar face playing a character dying from an overdose pretty earlt on. After checking the credits, I noticed that Jim went as James Carrey back then.

Samuel L Jackson in Coming to America

Steve Buscemi.

I've only ever seen the dude playing the main character in ONE movie. GhostWorld.

Outside of that, he's always just a rando side character. Yet everyone still knows who he is.

Chazz Palminteri as Hood #2 in the Last Dragon

This isn't exactly the same thing, but Matthew Fox played a paratrooper with no lines in World War Z. This was after Lost, so he wasn't an unknown by any stretch. He originally had a bigger part, but they changed the entire 3rd act in reshoots and his role was reduced to basically nothing.

Arnold Schwarzenegger in “The Streets of San Francisco”.

Also, does Jena Malone count? I saw her first movie, and wish I didn't. It was on late night TV. I'm not naming it or linking it because it has some really explicit, NSFL stuff in it and trigger warnings like you wouldn't believe. It was also briefly banned, and I happened to see it for sale on DVD with a big warning calling out what it briefly got banned for. You DON'T want to see it. (That said, she was not an extra, she was the most important character and basically the title character, it's just obscure for other reasons, like, most people would have never seen/heard of this film.) (And I don't want to talk about it, I like her acting but that film isn't one I will discuss.)

Anyway, I would later see her in Donnie Darko and Contact, and while she's never been a big star, mostly preferring indie flicks (she chose Contact because she always wanted to work with Jodie Foster, who is her role model), I'll still watch just about anything she's in. I think she's talented and I enjoy seeing her work. Kind of like Jake Gyllenhaal. I don't like a lot of actors for their acting, but when I find one I do, I tend to enjoy seeing them even if I don't like anything else about the movie.