(Non-professional) actors who did one memorable role in a movie or TV series, and not much else.

Frank Silva: a prop master who worked for several David Lynch projects, while filming Twin Peaks Lynch caught an accidental glimpse of him in the camera, and impressed by that Lynch made him into BOB. Apparently Silva never did any other roles on TV or in movies, and he died of AIDS in 1995.

Danny Lloyd: was chosen as the child star of Kubrick's The Shining among 5000 applicants. He did one more role in a TV movie after The Shining, but apparently he wasn't interested in an acting career, and hasn't been heard of since.

Other examples?

Charlie Bucket

You mean this Charlie Bucket?

Richardson in Deadwood.

Martine Lemaire in Diary of a Country Priest.

http://www.filmdope.com/Gallery/ActorsD/4803-5775.gif
Frank Doubleday - Two John Carpenter movies then off the radar (bit parts in Hill St Blues etc:) Love this guy though.

Oh shit! *Non Professional* whoops.

a lot of the kids in George Washington.

I put (non-professional) into brackets because usually the people who appear in one memorable role and then disappear are non-professionals, but it's okay to name professionals too if they fit the description.

Carrie Henn, who played Newt in Aliens, never acted again. I think she's a school teacher now.

I'm pretty sure the guy who played the Cowboy in Mulholland Drive hasn't acted in anything else either; he's a producer by profession.

Carrie Henn now:

http://lh3.ggpht.com/_krhe-i0hreY/SWczy-bQ4kI/AAAAAAAAB6k/JEvNsyZd5TA/s640/carrie_henn.jpg

http://en.wikipedia.org/wiki/Billy_Redden
http://www.proteinpower.com/drmike/wp-content/uploads/2006/12/deliverance.JPG

Jeffrey Fuggitt

pernilla allwin as fanny in fanny and alexander

http://www.altfg.com/film/wp-content/uploads/images/2004/12/fanny-and-alexander-dvd.jpg

bertil guve as alexander did a tiny amount further

Must be loads of kid roles this applies to

probably, but they're both very good

For sure! That was more in response to thread q than your post specifically