Favorite uncredited movie role for a well-known actor due to the fact that it would give away a plot point by mentioning them?

If a certain actor or actress is known for having played a certain character or a type or role in the past, their presence alone could influence the narrative. I think it never worked better than

Kevin Spacey in "Se7en," the secret was kept so well, remember kids once upon a time there was no internet, and we had no idea what to expect come the reveal of the killer. Not just that, the fact that Spacey knocked that performance out of the park and captured the creepy essence of his role just made the whole thing work magnificently well.

Also, I will always remember the first time Will Ferrell shows up in "Wedding Crashers." He was at his popularity peak by then and when he shows up, not only doesn't he make the film deteriorate, but it ends up being the funniest bit of the whole film.

Some of my favorites:

Kevin Spacey, Se7en
Alec Baldwin, Glengary Glen Ross
Gene Hackman, Young Frankenstein

Mark Hamill, The Force Awakens

Johnny Depp, 21 Jump Street

Tom Cruise, Tropic Thunder

Bill Murray, Zombieland

Robert De Niro, American Hustle

Will Ferrell, Wedding Crashers

Joe Pesci, A Bronx Tale
Natalie Portman, The Darjeeling Limited
Cate Blanchett, Hot Fuzz
Samuel L Jackson, Iron Man
Bruce Willis and Julia Roberts, The Player
Pamela Anderson, Borat
Charlie Sheen, Being John Malkovich
Neil Patrick Harris, Harold and Kumar Go to White Castle
Bob Barker, Happy Gilmore
Tom Cruise, Gwyneth Paltrow, Kevin Spacey, Danny DeVito, and Steven Spielberg, Austin Powers in Goldmember

Other instances when this happened:

Johnny Depp, Fantastic Beasts

Matt Damon, Interstellar, 

Matt Damon, Zero Theorem

Leonard Nimoy, Star Trek III: The Search For Spock

Boris Karloff, Frankenstein

Sean Connery, Robin Hood: Prince of Thieves

Edward Norton, Kingdom of Heaven