Did you know that Will Smith could have played Neo in The Matrix, Jack Nicholson turned down the role of Michael Corleone in The Godfather, and Adrien Brody was invited to play one of the four hobbits in the film adaptation of The Lord of the Rings, but he didn't want to come to the casting because he didn't get the idea of the book at all.
In any case, the history of movies could've gone completely differently if these star actors had accepted their invitations—but it's far from certain that they would have gotten as into their roles as Keanu Reeves, Al Pacino, or, for example, Elijah Wood. And believe me, there are many of these examples—like the ones found in this popular online thread.
More info: Reddit
#1
Rowan Atkinson as Mr Bean. You know you've nailed a role when most of the world knows you as your character's name, not your real name.

Image credits: tizz66
#2
Leonard Nimoy as Spock.
Zachary Quinto and Ethan Peck are fine in their own rights, but Mr. Nimoy's performance is what made Spock a cultural icon.

Image credits: Silvaria928
#3
Johnny Depp at Jack Sparrow.

Image credits: sherlock_jr
#4
Tim Curry as Frank-N-Furter!

Image credits: Honey_I_am_amazing
#5
SNAPE.

Image credits: FNc2nsZQLtOX24S4
#6
Maggie Smith as Professor McGonagall.

Image credits: Phillimac16
#7
Ian McKellen - Gandalf.

Image credits: Jay-Dee-British
#8
Gene Wilder as Wonka.

Image credits: Karmacoma77
#9
Hugh Laurie as Dr. House.

Image credits: JBmadera
#10
Christopher Lloyd nailed it as Doc Brown in the Back to the Future movies.

Image credits: Itisd
#11
Arnold Schwarzenegger as The Terminator!

Image credits: proclubs24
#12
Tom Hanks as Forrest Gump.

Image credits: punk0mi
#13
Let’s just say a Misery remake would be utterly pointless.

Image credits: NeitherSparky
#14
RDJ as Iron man/Tony Stark.

Image credits: Brush_bandicoot
#15
Julie Andrews as Mary Poppins, to the extent that my ex doesn't even know her real name. Anytime he sees Julie in a movie, he calls her Mary Poppins (writing that does not do justice to hearing this British character's name in a Salvadoran accent).
(honorable mention: also as Maria Von Trapp, much nicer than the real MVT).

Image credits: Tejanisima
#16
Christoff Waltz as Hans Landa.

Image credits: Nicetoyourface87
#17
Obligatory Heath Ledger as The Joker response
Him copying Tom Waits was a stroke of genius.

Image credits: InvadeTheUSA
#18
Only Mike Meyers could make Austin Powers work.

Image credits: thatlookslikemydog
#19
Bryan Cranston in Breaking Bad.

Image credits: No-Wear-3028
#20
Jack Gleeson as Joffrey in Game of Thrones. Never wanted to punch a character so bad….

Image credits: Ark2226
#21
Matthew Lillard as Shaggy Rogers. So good he’s ended up voicing Shaggy in lots of further adaptations of Scooby Doo.

Image credits: _disgustingly
#22
Macauley Culkin as Kevin McCallister.

Image credits: TheThreeRocketeers
#23
James Gandolfini - Tony Soprano.

Image credits: DarkPasta
#24
JK Simmons as JJ Jameson.

Image credits: AContrarianDick
#25
Charles Dance as Tywin Lannister dude owned every scene I could see him as the only one unimpressed by the Night King.

Image credits: poop_truck1226
#26
Patrick Stewart as Jean-Luc Picard
Jason Sudeikis as Ted Lasso
Jack Nicholson - Jack Torrance (The Shining).

Image credits: bevymartbc
#27
Steve Carell as Michael Scott.

Image credits: melissapurtle
#28
Bryan Cranston as Walter White according to Vince Gilligan
I’d say you couldn’t replace Bob Odenkirk as Saul Goodman either.

Image credits: learn2cook