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Some characters that had around 15 minutes or less in a film but had a huge impact on it:
The Wicked Witch of the West (Margaret Hamilton) in The Wizard of Oz (1939)
'Boo' Radley (Robert Duvall) in To Kill a Mockingbird (1962)
Fantine (Anne Hathaway) in Les Misรฉrables (2012)
Maybe (not sure if any are 15 minutes or less):
Blofeld (Donald Pleasence, Charles Gray and others) in various James Bond films.
"Mike Evans" in "Three Body".
@nealfig
Hannibal Lector (Anthony Hopkins) in Silence of the Lambs (1991)
Lynn Bracken (Kim Basinger) L.A. Confidential (1997)
@Animeraider Definitely Hannibal Lector! I can't remember L.A. Confidential but will watch it again.
@nealfig Warning: strong acting performance from a known sexual predator. Not Kim Basinger.
@nealfig
The "Xenomorph" in "Alien".