I knew him first as Max’s (Peter Riegert’s) Scottish sidekick Oldsen from 1983’s Local Hero. “You say the darndest things Marina” (a whole generation of kids will know him first as the villain’s sidekick in Paddington. “He used to be a Doctor Who?!”)
There are tons, but these come to mind immediately:
I first saw Brad Pitt in Thelma and Louise, but really noticed him (enough to know his name) as the freakyscary killer in Kalifornia. It took me a long, long time to figure out why people kept calling him a “pretty boy.” I guess it’s from A River Runs Through It, which I’ve still never seen.
There was a time in my life when I watched soap operas and I knew Demi Moore as Jackie Templeton on General Hospital. I loved her voice. She stood out from everyone else.
This is more recent, but I discovered Jennifer Lawrence in Winter’s Bone (which got her her first Oscar nomination and the role of Katniss in The Hunger Games), but she had been in a few movies before that that I should have seen.
I first saw Thor’s Stellan Skarsgård (as well as Emily Watson) in 1996’s Breaking The Waves. It was Emily Watson’s first film role, but Skarsgård had been in tons of things prior.
I first saw Oscar winner Eddie Redmayne in 2006’s The Good Shepherd, directed by Robert De Niro. He played such a slimy scumbag I hated him until The Theory of Everything.
I first saw Benedict Cumberbatch in 2006’s great movie Amazing Grace. He’s listed as being in Starter For 10 with James McAvoy, which came before Amazing Grace and we saw it in the theater, but I don’t remember him. I’ll have to watch it again. I’m sure I’ll say “Oh yeah, that guy!”
Before the Star Trek reboot I had seen Chris Pine in a tiny fun little movie called Bottle Shock. Two years before that he was in a movie I really liked, Smokin’ Aces, but I don’t remember him in it.
Speaking of the Star Trek reboot, after seeing it I made it a point to look up who played just-born Kirk’s dad in the movie, because I thought the scene and the acting was amazing. It was Chris Hemsworth. I’ve kept a (very close) eye on him ever since.