For me, Jewel Staite was a draw to Firefly, because I remembered her from the first season of Nickelodeon’s Space Cases. She played Catalina, and had rainbow hair.
Dawn from Buffy is always Little Pete’s neighbor friend with the perpetual cast from The Adventures of Pete & Pete. And a guest spot on Clarissa Explains It All. Yes, I grew up in the early 90s; why do you ask?
I don’t remember that, but her IMDB listing includes three *St. Elsewhere *episodes in 1986-87.
I did see Kathy Bates in an absolutely riveting performance in 1985 at Actors Theater of Louisville. It was part of the Humana Festival. The play was Two Masters by Frank Manley.
I remember Matt LeBlanc as Kelly’s equally hot, equally dumb boyfriend on MWC.
I also took notice of Paul Giamatti as the bellboy in My Best Friend’s Wedding. The scene used to be on YouTube but seems to have been taken down. Anyway, it’s one of those “Jeez, this guy’s good; what else has he been in?” moments.
I remember him in Platoon, but the guy who stood out to me was John C. McGinley as Sergeant O’Neill. I remember seeing Office Space and thinking “Who knew Sgt. O’Neill’s name was Bob?”.
I also recall Tom Cruise in Taps, and Sean Young & Judge Reinhold in Stripes.
I caught what you did. 
He eventually became lead singer for the Dead Kennedys. Really. See here.
Shelley Long in local ads. She was the most beautiful woman I’d ever seen on TV and she made an impression.
She lost some of it by the time she was on Cheers. She was really that gorgeous.
I saw Jay Leno on an episode of Good Times. He played a patient in a VD clinic who gave the educational infodump about the importance of being tested. He really stood out. Years later, when I saw him doing stand up on some other show, I wondered if his blood test came back negative.
Was Billy Crystal already known when he was on Soap? If not, then I claim that.
I always took it that that was what made him a star.
So does Fast Times at Ridgemont High win the award for casting the largest number of soon-to-be stars in movie history?
American Graffiti would be a contender. It even had Harrison Ford pre-Star Wars!
Tim Robbins was in a low budget T & A comedy Fraternity Vacation.
I always laugh because Robbins always acts so intellectual and pompus in interviews. There’s a scene of Robbins in Frat Vac where he’s watching these two bimbos take off their bikinis. He’s got the funniest crap eating grin on his face.
Ah, and I recall Spalding Gray in a low-budget soft-core porno film entitled The Farmer’s Daughters. He was running around screwing a chicken. Really.
The plot according to imdb.com: “After the farmer’s daughters gang rape the farm hand Fred, three escaped convicts arrive and have their way with members of the family. Fred surprises the convicts, only to replace them in the family’s victimization.”
I saw Meat Loaf in The National Lampoon Show (not “Lemmings”) when it toured college campuses. He apparently came in to replace John Belushi. He sang the opening song and acted in “The Rhoda Tyler Moore Show”. This was pre-Bat out of Hell, and pre-Rocky Horror.
And I didn’t see him, but a friend who had a job at an Indianapolis TV studio in the early 1970s told me about the hijinks of their weather man, who I’m convinced must have been David Letterman, long before he hit the national scene
Diner could be a candidate, too. At the time I saw it in the theater, the only cast member I’d ever seen was Daniel Stern - he’d been in Breaking Away. So Steve Guttenberg, Mickey Rourke, Kevin Bacon, Tim Daly, Paul Reiser, and Ellen Barkin, were pretty much unknown to me. (I’d seen Mickey Rourke as the arsonist in Body Heat, but didn’t remember him when I saw Diner.)
You can bet that after Diner, I remembered every single one of them!
I remember David Duchovny as the cross-dressing (or was it transsexual?) agent in Twin Peaks.
Cate Blanchett in Paradise Road. She was an ‘unknown’ then compared to the rest of the ensemble - Glenn Close, Jennifer Ehle, Frances McDormand, Julianna Margulies, but it was Cate’s performance that made the biggest impression on me.
Ellen DeGeneres in the short-lived sitcom Laurie Hill.
Sarah Jessica Parker in Equal Justice (though this was after Square Pegs).
Kim Cattrall in Police Academy.
malcolm in the middle featured 2 future hotties:
cynthia the ugly duckling kreylborn was portrayed by tania ryamonde - alex rousseau from Lost
Jessica the frizzie haired babysitter from later seasons was Hayden Pantierre - claire on heroes.
I remember the first thing I saw Jack Black in was Bob Roberts. He struck me as having the creepiest eyes ever.
I recognized Tim Robbins from The Quarterback Princess (with Helen Hunt) and The Sure Thing when Bull Durham came out.
There was a British TV movie about an advertising agency that played on A&E in the mid-80’s called Honest, Decent, & True that had a guy named Gary Oldman in it. I’m pretty sure I saw it before I saw Sid & Nancy. I watched it because it had Vyvyan Basterd from The Young Ones. Richard E. Grant was in it too. There was also a short film GO did about soccer hooligans called The Firm that I saw somewhere around then. I vividly remember a scene where his son eats a razer blade.
Christian Slater was in a ABC weekend special. I seem to remember him sitting on a horse at the end and offering a girl a ride.
River Phoenix was in an after school special about dyslexia. I think people at his school figured out he had it after he wrote graffiti on some lockers… or the people at his school knew who did the graffiti because of the way it was written, which would make more sense. I mostly just remember him doing the graffiti… He was also on an episode of Family Ties.
Sean Astin was in a TV movie about child abuse before he did The Goonies (and LotR). I remember him dropping a textbook into water that was puddling around a curb. It was the same textbook that I had (I think it was for science or math).
Matt Dillon was in Over the Edge, My Bodyguard (with Adam Baldwin and Joan Cusack), and Little Darlings. They all seemed to show up on cable around the same time in the early 80’s. I watched Over the Edge and My Bodyguard over and over again and they’re still two of my favorite films.
Ben Stiller was in Empire of the Sun with a 12/13-year-old Christian Bale. I hated him for bullying Jim/CB.
Anthony Michael Hall was in a Tom Sawyer/Huck Finn TV movie called Rascals and Robbers and in the movie Six Pack before he did all his John Hughes films.
Robert Downey Jr. had small parts in Firstborn and Weird Science.
Molly Ringwald was part of the original cast of the Facts of Life. I remember recognizing her and a few of the other girls from the New(ly canceled) Mickey Mouse Club.
I remember her in Mannequin from shortly after that. I did not see the movie, but they played scenes from it in that Jefferson Starship music video, since they had a song in the film. She was smokin’ hot!