I was reading the wiki on the film “Groundhog Day”, pretty sure for the first time.
I was a little surprised there was some contention on the screenplay (Rubin and Ramis) yet also that Ramis and Murray, who had worked together since 1974 and had a good relationship and been in several films together - yet Ramis wasn’t all that keen about casting Murray who while not being a Chevy-Chase, can be somewhat difficult to work with. Yet his work - that is what shows up on the movie reels - is often superb and it’s difficult to imagine someone else in the role. Tom Hanks was offered the role, so was Michael Keaton - yet they declined.
A single film might be too simple. Three films can be considered at least a starter in a “body of work” where you or a casting director would think right off, “Tom Hanks is the only guy who can play this role.”
So it is three films that an actor appeared in that you would not think “meh, Mel Gibson could have done that.” Or that in some other reality it would not have mattered if this actor appeared in the films he or she appeared in. I suppose the phrase “they owned the film” applies if that helps.
Remakes/reboots pretty much do not count. And if Zero Mostel is your Tevye on Broadway in “Fiddler on the Roof” you get Chaim Topol in the film.
The Monty Python troupe is right out. We’re not even going to consider replacing any of them.
Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three.
It helps to be the screenwriter, director and star (good luck doing that now)
Great roles by great actors whom it would be (almost) inconcievable to cast another actor in:
Orson Welles - Citizen Kane, The Third Man, Touch of Evil
Jack Lemmon - Mister Roberts, Some Like it Hot, The Apartment
Jimmy Stewart - It's a Wonderful Life, Mr. Smith Goes to Washington, Rear Window (honorable mentions for Rope and Vertigo)
Gene Wilder - The Producers, Young Frankenstein, Stir Crazy
Bill Murray - Stripes, Ghostbusters, Lost in Translation
Harrison Ford - Han Solo/Star Wars, Indiana Jones/Raiders, Blade Runner
Robert Downey Jr - Less than Zero, Tropic Thunder, Iron Man
Tom Cruise - Risky Business, Top Gun, The Color of Money (lots of hon mentions)
Nicolas Cage - Raising Arizona, Leaving Las Vegas, Wild at Heart
Keanu Reeves - Bill & Ted's Excellent Adventure, My Own Private Idaho, John Wick/series
River Phoenix - Stand By Me, My Own Private Idaho, Litte Nikita (with Sidney ...)
Sidney Poitier - Blackboard Jungle, Guess Who's Coming to Dinner, In the Heat of the Night