Good to know the objective better. It’s not coming to me just now, but I remember reading something very recently (perhaps heard it on TV) about such a role. Whoever this was is felt as a presence by way of other characters’ references to him (or her) but whose actual screen time is very minimal, a matter of just a few minutes over the entire movie. It was, no doubt, a big name star. I’ll be trying to remember who it was and what the movie was. I don’t think it was Brando in Superman or Apocalypse Now, but it was that sort of thing.
I don’t know if it was iconic, but in the pilot episode of The West Wing, Martin Sheen’s President was only in one scene, and the producers only intended him to be an infrequent part of the show, only showing up a few times a year. The show was originally going to deal entirely with the West Wing staff. Then, when they were filming the pilot, they were blown away by his performance and asked him to join the regular cast.
Silence of the Lambs is a great movie with great performances by everyone involved, but I think Anthony Hopkins stole the show with his first appearance in the jail as Hannibal Lecter. He only got 16 minutes of screen time in the movie, but it was enough to win him an Oscar for Best Actor.
While I don’t think she deserved an Oscar for it, Dame Judy Dench’s performance in Shakespeare in Love was indeed fantastic. Something like 7 minutes of screen time, but a great 7 minutes it was. She’s just magnetic.
Bill Murray in “Zombieland”.
Samuel L. Jackson’s “Ezekiel 25” Speech from Pulp Fiction.
Yeah, but he is awesome in the whole movie. If that had been his only scene, though, it’d be amazing.
Chris Rock in I’m Gonna Git You Sucka.
“You got change for hundred?”
“Nobody fucks with the Jesus.”
“I love the smell of napalm in the morning.”
uh, what Snickers said.
Great! That’s the one I was I was trying to recall. We saw a “making of” special on it just this past month and that point was stressed.
Thanks for saving my mental capacities, Barkis is Willin’.
Very interesting question; I’m going to be pondering this all day.
My first nominee -
Vincent Schiavelli as the Subway Ghost from the otherwise terrible film ‘Ghost’.
“Pardon me, stewardess, I speak jive.”
Absolutely!
It may be that my teen idol worshiping self couldn’t get enough of him but
Giant (1956) - James Dean Strikes Oil
is my favorite scene(s) of the boy.
Not a film, but Carl Weathers in “Arrested Development”.
“Whoa, whoa, whoa. There’s still plenty of meat on that bone. Now you take this home, throw it in a pot, add some broth, a potato. Baby, you’ve got a stew going.”
I also think Tom Cruise pretty much stole the show with perhaps his greatest performance ever when he portrayed movie producer Les Grossman in Tropic Thunder. I didn’t even know he was in TT until I saw it and even then it took me a second to recognize him. He’s only got a couple scenes, but they’re the funniest in the movie.
Yo, Diogenes the Cynic, should we be counting things like Drew in Scream (I) (1996) and Seagal in Executive Decision (1996) where the mere fact that they didn’t hang around long was actually a plus for the movie? 
FWIW, this thread’s title is ideal!
Other Nicholson greats:
Five Easy Pieces: the restaurant scene when he’s trying to order toast. “Hold it between your knees”.
The Shining: “Here’s Johnny!” scene.
He was the main character in that.