I don’t think I can agree with this one. You may have first discovered Lithgow in Third Rock due to your age but he was no unknown. It was a case of a well-established movie actor (he already had two Oscar nominations) taking the lead in a TV series. To me, the breakout star on Third Rock was Joseph Gordon-Leavitt.
It has to a generational thing. For us old farts, we recognized Lithgow as a breakout in The World According to Garp.
When 3rd Rock From the Sun was announced (and I’d probably seen a preview or two) but not yet on the air, I was discussing it with a co-worker. He’d seen Lithgow in Terms of Endearment and wasn’t sure he’d be able to play an over-the-top wacky alien. I knew him from Garp and Buckaroo Banzai and wasn’t sure he’d be able to tone his performance down enough for TV.
Saw him several years ago reading one of his children’s books to a bunch of kids, and he was brilliant at that, too.
You could actually argue that Mcgill’s one of the most prolific character actors working today.
I knew Anderson did Stargate but I would’ve thought he’d go on to feature film roles
I was going with the OP’s description of actors that bucked our expectations. For me personally that was Lithgow. It may be obvious for someone who followed his career before Third Rock but as someone with only that as their exposure, I was amazed to read up on all he was involved with and his incredible range. The guy can ACT!
I think once Joseph Gorden-Leavitt got over his awkward teens he had a great platform to be big. He paid his dues in Third Rock showing he could commit to the bit and be reliable. I didn’t expect it, but he certainly deserves it!
On the polar opposite, thank god French Stuart didn’t make it.
I never thought HHH would amount to much in the WWF/E when he debuted as some rich snob
Old fart data point…Manhattan Project was what cemented Lithgow into my memory as a superstar that I knew would get jobs forever.
Mark Wahlberg - Turns out Marky Mark could actually act.
Some of these names mentioned I was just thinking about. Most of mine have to do with music rather than actors.
Pink: When I first heard her in 2000 I was sure that she would disappear soon. In fact she has been very consistent for nearly 15 years, with a single most years since then. Heck she does not get much press but she has been releasing more material than the likes of Beyoncé and other big names.
Maroon 5: In 2002 with the song “Harder to Breathe”, they kind of fell off. It was from the album “Songs About Jane”, which took off in 2004 making it a sleeper hit. So the band and the album got off to a slow start.
Rihanna: Upon hearing her first song “Pon the Replay” in 2005, I thought she would die quickly career wise. But she has had a#1 single every year from 2006-2013.
Lady Gaga: She has been very successful and seemed to come out nowhere, and will be an icon. Yet she likely won’t see the same success she had in 2009-2011. Many predicted she will be the next Madonna, but I think she will on par to Cyndi Lauper. Always will be remembered as an icon, but the hey day was short.
My current predictions, and feel free to agree or disagree.
Lorde: I think her name will float around and she will may release another successful album. However in the long run she will not have longevity and will disappear in obscurity with “Royals” being her only identifiable hit.
Adele: She will never again experience the massive success of “21”.
Go on, laugh at me…
When I first saw Piper Perabo in Coyote Ugly, I thought that she would blow up into a huge star. The movie was a colossal turd, of course, but I just thought that the camera loved her and she’d go on to an A-list career.
Bryan Cranston.
I enjoyed Malcolm in the Middle as a mild diversion back in the day, and thought the dad was fun/goofy.
Never did I dream he had anything like Walter White within.
mmm
I expected big things from a post-BACK-TO-THE-FUTURE Thomas F. Wilson.
Old fart here. 
Garp was a breakout vehicle for many – it instantly made Glenn Close my favorite actress and Robin Williams my favorite actor. Of course Robin was well known for his comedy in Mork and Mindy, but Garp revealed his serious acting talent. And it was Glenn Close’s very first film.
I don’t know why Leonardo got such a bad rep. Must be Titanic. His track record before that was quite good.
Gotta mention Phillip Seymour Hoffman. I first saw him in “Scent of a Woman”. I was wondering how he even got THAT part, let alone thinking his career would reach the heights it got. He became one of my favorite actors.
Never appreciated Jimmy Fallon. Not as an actor, comedian, host, or anything. But now he’s host of the Tonight Show.
After her initial success, I thought Debbie Gibson would be huge. C’est la vie.
Understood, but you didn’t have to follow his career to know exactly who he was. Before 3rd Rock he was already well known among all demographics. Grown-ups knew him from World According to Garp and Terms of Endearment, teens knew him from Footloose and The Twilight Zone, and kids knew him from Harry and the Hendersons.
Plus, half the country (100 million viewers!) saw him in the tv movie The Day After. He wasn’t the lead, but he was a featured player. For context, that’s a Superbowl size audience.
I still remember Hocus Pocus because of the Cincinnati Reds 1975/76 seasons. They made a choreographed commercial to it that was awesome.
I was surprised by Cher’s acting career. I thought she did surprisingly well.
I agree. I’m one of the few who think she’s a better actress than singer and was disappointed she seemed to gave it up.
Count me in, as well. She was great all the way back to the Sonny & Cher Comedy Hour–good timing, excellent deadpan, not overbearing but didn’t get lost in the crowd, either. What’s her big line in Moonstruck?–“Aw, Ma, I love him awful.” Killer.