Cool Hand Luke is old enough that everybody in the “before they were stars” category in that movie have since become the stars they weren’t in the late 60’s, but among the ones who were cast as extras or bits were:
Joe Don Baker
Harry Dean Stanton
And I have heard that several stars-to-be were in the chorus of sailors in South Pacific but I have yet to find a convincing list of those guys.
The one that strikes everyone is Sylvester Stallone as a subway punk in Woody Allen’s Bananas. Nowadays, he’s impossible to miss – a full-face shot as he comes on. But when the film first came out, nobpdy knew anything about him, and he was ananymous. His later fame completely changes that scene.
A low-budget “horny kids go to the beach” teen comedy called Private Resort featured before they were famous performances by Johnny Depp, Rob “Northern Exposure” Morrow, and Andrew Dice Clay. (Today the film is famous for containing an almost unrecognizable Depp’s only nude scene [Morrow’s too for that matter].)
The mostly forgotten sitcom Benson featured Jerry Seinfeld in a supporting role (for one season) and Ted Danson in a recurring role (as the husband of one of the characters) before they were famous.
The Empire Strikes Back had b.t.w.f. appearances by John Ratzenberger (Major Derwin) and Treat Williams (Cloud City Trooper), but I have absolutely no idea where they appear (other than Treat Williams is obviously in the Cloud City scenes).
Mike Farrell of MAS*H has a one sentence walk-on as a bellman in The Graduate.
Alan Alda played the Forrest Gump-like son of Sorrell “Boss Hogg” Booke (who basically played a prototype of Boss Hogg) in Ossie Davis’s Purlie Victorious. Sorrell Booke had a semi-nude sex scene before he was famous in The Other Side of Midnight.
Carroll O’Connor had supporting roles in Cleopatra (as Casca) and as Julie Andrews’ scenery-chewing gruff father in Hawaii (a movie that also featured Bette Midler in non-speaking extra roles as a missionary’s wife and as a topless island girl swimming out to meet the ships).
I’ve recently seen Rob Reiner in 40 year old repeats of The Andy Griffith Show (playing a printer’s assistant) and The Beverly Hillbillies (playing a college hippie who Jethro hangs out with). The Hillbillies ep. actually had a funny line- Jethro tries to incorporate hippie/activist speak and calls Uncle Jed a “honkey” and a “whitey” when he gets mad, to which Reiner Hippie tells him “That’s the wrong activist group Jethro!”
I had to Google that name because I’d never, ever heard of her (wasn’t sure it was a her and not a him). The picture didn’t help. Apparently she’s in a TV show called Heroes, which I have heard of but never seen. She’s famous to some people, I guess, but I wouldn’t notice her walking down the street.
For me, it was Sylvester Stallone in Death Race 2000. Released pre-Rocky, Stallone plays just another part opposite such “stars” as Simone Griffith, Mary Woronov, Roberta Collins, and Martin Hove. To give Stallone credit, he does a good job of it; but it really is a B-picture, and has nothing to indicate that Stallone would eventually be anything other than a B actor.