Bridges was not an unknown before Stay Hungry. He had a run of movies before that, including The Last Picture Show and Thunderbolt and Lightfoot, for which he had been nominated for an Oscar. Not all of his films were big box-office successes, but I was certainly aware of who he was at the time. It didn’t hurt that he was the son of a famous actor, Lloyd Bridges.
Tim Robbins in Top Gun.
Meg Ryan, also.
Before guesting on Family Ties, Geena Davis was a main character on the series Buffalo Bill, also starring Dabney Coleman, Joanna Cassidy, and Meshach Taylor, which ran for two seasons. She was already something of a name based on her reputation on that critically acclaimed series.
This is who I would pick (and how, he was and still is, cute), but the movie I’m thinking of is “Adventures in Babysitting”. He played Thor. Both movies were in 1987, but I don’t know which was first.
I just spotted a very young Bruno Kirby Jr in an episode of Columbo. Bruno’s dad Bruce (nee Bruno Sr) played the gullible Sgt Kramer in the Columbo shows.
Before they became rich by writing trashy novels, both Jackie Collins and Jacqueline Susann appeared in several films and tv shows.
Brittany Murphy was the younger daughter, Brenda, on the Dabney Coleman sit-com Drexell’s Class. She had also appeared on an episode of Murphy Brown.
Okay, but do I get any points for Anthony Hopkins, Daniel Day-Lewis, Bernard Hill, and Liam Neeson?
He’s also the guy that Geena Davis hooks up with in Thelma and Louise.
And Floyd the stoner in True Romance.
It’s sometimes hard to tell when people are joking, and when there’s a generational divide, or something.
Anthony Hopkins? You might have cited The Lion in Winter from 1968 as “Before they were well known”, but Hopkins had already starred in The Elephant Man and Magic and a host of other films well before 1984.
And I’m kinda annoyed at people thinking Leslie Neilsen was unknown before Airplane!. They got him because he was a familiar face – and not just from Forbidden Planet. I certainly knew who he was before then. Look him up on the iMDB.
For all his many pre-Airplane roles, the one that I most remember was the “Night Gallery” vignette in which he played an old soldier of fortune who was being offered some large some of money (by Fritz Weaver) to spend a night alone in a “house of terrors.”
But now that I rememeber “Night Gallery,” if featured some early appearances by Lindsay Wagner and Randy Quaid, neither of whom I would have recognized had I not known it.
I happened upon a young Jude Law on an episode of* Sherlock Holmes* a couple of months ago (the series with Jeremy Brett). He was onscreen for mayyybe 60 seconds, and had only a couple of lines.
Ian McShane was in a really early episode of Magnum P.I.. Doesn’t completely look like him, but the voice is a dead giveaway - he sounds just like he did as Tai Lung.
Dude, I’ll hook you up with 200 points. 
In “The Execution of Private Slovik,” (1974) starring Martin Sheen, there is a scene at Slovik’s wedding reception in which a young boy is shown wandering about. The boy was one of Sheen’s sons, Carlos, better known as Charlie. Charlie is now known for his strange ramblings in interviews and his own internet show, I believe.
Carrie Fisher plays one of Warren Beatty’s conquests in shampoo, well before Star Wars. She was even caricatured in the Mad parody of it, well before SW came out.
One of the weirdest was an appearance on a 1950 TV show called Little Theater of a playlet called “The Bitter End”, about a man trying to commit suicide and failing. It wasn’t until his last line “Well, whaddaya know! i couldn’t do it!” that I realized who the hell it was. He said “Well whaddaya know! I finally got the last word” exactly the same way at the end of an episode of Star Trek. It was a much younger De Forest Kelley, looking different enough that I hadn’t recognized him. The show appears with the bogus title Suicide Theater on various “Bloops and Blunders” and “Star Trek Bloopers” compilations.
Here’s one appearance of it:
I note, by the way, that it isn’t listed on his iMDB page. But it definitely exists.
I’m watching Robocop 1 now, and did you know that Leeza Gibbons was also the newscaster in the film?
Also, for a non-human appearance, one of the first issues of Games Magazine was in National Lampoon’s Vacation.
We had that issue at home, before the movie. Famous skyscrapers on the cover, right? Then I saw the movie and there’s the whole Beverly D’Angelo thing… After the film, I was a little bit mixed up about buildings and boobs for a month or two, but I think I figured it out.
Yeah, puberty was fun.