He was a standout in Dominick and Eugene, playing the mentally challenged brother of Ray Liotta.
I think you’re overstating that role a bit, I think he has like 4 minutes of screen time in Pulp Fiction.
Heath Ledger and Cesar Romero would respectfully disagree.
Seems to me that a lot of these fall into two categories- child/teen actors who had one big role, and then never really found their footing, or character/supporting actors who had one big role, and then went back to character acting. Or some sort of found their names in dual-star or ensemble casts, not as sole protagonists. I’m thinking about Cuba Gooding Jr. and Tom Hulce here- both of them are best known for movies where they weren’t the sole protagonists, and were cast either as part of an ensemble, or as part of a pair of stars.
I would say Mark Hamill stands out to me; he basically came out of nowhere, starred as Luke Skywalker, and then faded into voice acting semi-obscurity.
Why do you have to diss Cock Knocker like that?
He’s a God of voice acting. Which is niche, but hardly obscurity, semi- or otherwise… his series of Joker-reads-Trump videos were a big hit online.
Michael Sarrazin was the new hot boy on the street when he made The Flim Flam Man with George C Scott and Sue Lyon (who also might qualify for this). He worked continuously for another 40 years, mostly bit parts and TV appearances; some highlights included The Reincarnation of Peter Proud, The Loves and Times of Scaramouche(he played the title roles in both of these), and he played Frankenstein’s monster on some TV remake in the 70s, but never really regained his momentum. He had a small part in On the Road nine years ago and apparently died right after that.
Marjoe Gortner, a former child evangelist, starred in Marjoe, an expose of the tent preacher racket, in 1972. His subsequent career, which lasted about 23 years, kind of defines “diminishing returns.” He starred in Starcrash, the kind of crappy sci-fi vehicle people stopped making after Star Wars, right after Star Wars was released, and kind of spiraled downward from that. He was also on the rebooted version of Laugh-In in 1977-78.
Jewel (the singer) was cast in a leading role in a 1999 film, Ride with the Devil. Hearing that she was cast as the lead in a major motion picture made me think she might become one of those superstars that’s famous for both her music and her acting, like JLo or Demi Lovato. But that never happened.
Stacey Dash also comes to mind. She starred in clueless along with a bunch of other actors who went on to some pretty successful careers (Paul Rudd and Brittany Murphy, for example). But I think people only know her as an actress from Clueless. (I would put “news contributor” in a separate category from acting.)
It’s hard to know what exactly fits the OP, since both of those people did make other appearances on TV. But anything else they were offered seems incredibly minor in comparison.
You neglected to mention his tour-de-force performances in Bobby Jo and the Outlaw, Food of the Gods, and American Ninja 3: Blood Hunt. 
Someone who I was thinking of was Jennifer Hudson. She got rave reviews for her role in Dreamgirls and won the Oscar for Best Supporting Actress that year. She’s certainly had plenty of acting work since then, but nothing that seemed to rise to the level of that initial promise.
Obscure for the star of the fourth highest grossing film of all time (adjusted for inflation), when you think that he did not star in ANYTHING of any real note after Jedi, and before “The Force Awakens”.
I don’t doubt it was by choice; I can’t imagine that the attraction of being a voice actor and sometime actor in low-budget sci-fi, and raking in the Star Wars residuals, wasn’t huge. He had his “fuck you money” so to speak, and took advantage of it I’m assuming.
Another one who I think falls into the same category, although even more dramatic is Sam Worthington. Star of Avatar, and then not much else of note besides playing Perseus in some IMO, bad Greek mythology movies.
Craig Charles played the lead in one of the shows of my childhood, Red Dwarf. Though its still being made, or at least was until recently, its not what it once was (there is a reason British shows don’t normally have too many seasons)
But he’s done nothing else in terms of fiction on TV (he hosted the UK version of Robot Wars), though has had a fairly successful career as a DJ on BBC radio. He had some well publicized (thanks to scumbag UK tabloids) struggles with drug abuse.
When I think of Sam Worthington I also think of Taylor Kitsch. He gets steady work but never got the ‘star’ role that studios tried to give him.
The only reason Christopher Jones had an acting career was his resemblance to James Dean. Mr. Jones was in two seasons of a TV Western before gaining cult status and PR push as the star of Wild in the Streets (1968). He hit the bigtime in The Looking Glass War (1970) - a John Le Carré adaptation that flopped - on the basis of which legendary director David Lean unwisely cast him as the lead in Ryan’s Daughter…not realizing that Mr. Jones had been dubbed in the previous film. Mr. Jones was also dubbed for Lean’s film, proof that it takes more than looks to make an actor. Apart from a supporting role in the obscure Mad Dog Time in 1996, Mr. Jones never acted again (not a great loss, imo).
I think of Taylor Kitsch often, I think the studios were trying to make him happen ten yrs ago and the works never panned out. Like that scifi film? or even that Battleship one.
Or, they won’t be able to find any other project to define them. Like many years ago the stars of MASH , some stated not listing MASH on their resumes, because 1)it’s dated 2)nothing else would be as big as MASH.
Two ultimate one-hit actors are Harold Russell and Haing S. Ngor, two non-professional actors who won acting Oscars.
Russell lost his hands during World War II, and was cast a couple years later in The Best Years of Our Lives, about servicemen returning and adjusting to civilian life. He was awarded an honorary Oscar for his inspiring performance, and won for Best Supporting Actor, too. He’s the only person to win two Academy Awards for the same performance.
Speaking of MASH, Jamie Farr was spat out the bottom of the game show industry after his run. Same with Gary Burghoff, who pretty much made a career (both movie and TV series) of the show, then ended up doing game shows. I guess one could argue that this doesn’t really fit the requirements of the OP, however, as neither of them would have been thought of as star material.
Jon Hamm, though, seems like quite possibly the most perfectly-positioned TV actor ever to springboard off his award-winning role as a handsome leading man with great comic timing, only for years and years of Hollywood ‘star’ treatment to — er, not quite ensue.
I think the first time most of us noticed Rutger Hauer was in Blade Runner (a charismatic performance topped off with that magnificent soliloquy). He was in a lot of films after that but rarely rated much higher than a B list action staple.
Fwiw, Hamm has been repeatedly asked to be the big star in a bunch of stuff, but he’s always said no. It’s one thing if you’re trying to break through and fail, it’s another thing if you’re apparently not interested.