Would Cylops have been nonexistant in X2 and uncerimoniously dumped in X3 if it had been Jim Caviezel instead of James Marsden?
IIRC, Cyclops' death in
X-Men: The Last Stand had less to do with the studio disliking Marsden and more to do with Marsden having a scheduling conflict with
Superman Returns. Also, IIRC, Famke Janssen was originally supposed to play Miss Kitty in
Superman Returns but couldn't because of her obligations to
X-Men: The Last Stand, so Singer replaced her with Parker Posey.
Eric Stoltz had filmed quite a bit of Back to the Future before it was decided Michael J. Fox would be better for the role.
Which caused further ripples with the film when they had to recast Marty McFly's girlfriend because the women on the crew objected to Marty having a girlfriend who was taller than him.
Also, had Michael J. Fox been cast as Marty to begin with, Biff probably would have been played by J.J. Cohen. He was a suitable bully for Fox but looked too short and unimposing next to Eric Stoltz. So when Stoltz got the role of Marty, the role of Biff went to the much taller Thomas F. Wilson. Meanwhile, J.J. Cohen got the consolation prize of playing one of Biff's goons.
Paul Newman only landed the lead in Somebody Up There Likes Me because James Dean died in an accident (and hopefully went 'Up There' ...). Steve McQueen, who had a small role opposite Newman in SUTLM) was supposed to co-star with Newman in Butch Cassidy & The Sundance Kid but couldn't agree billing with Newman, so Robert Redford scooped the role and became a star. Redford, in turn, was the first choice to play Benjamin Braddock in The Graduate (BB in the novel is physically like the former Sundance Kid) but again, someone felt that no-one would believe him as an awkward, sexually inexperienced nerd, so the 30 year old Dustin Hoffman was sent on the road to stardom as the teenager.
And had Hoffman not been busy shooting
The Graduate, Mel Brooks would have cast him as the deranged Nazi playwright in
The Producers.
Old school: George Raft famously passed on what would be Bogart's two greatest roles-- Casablanca and the Maltese Falcon. (Though in Casablanca's case, it may be apocryphal.)
According to some documentary I once saw, damn near every one of Bogey's famous roles was one that had been turned down by George Raft or Edward G. Robinson.
Harrison Ford in the Bob Hoskins role in Jessica Rabbit (might have been interesting to see the 'tec as a more Bogart-esque or even Mike Hammer type, but I think Hoskins suited the part better)
Maybe Ford thought it would be too much like a comedy version of his role in
Blade Runner.
Sean Connery, then Brian Cox were named to voice Aslan before [Liam Neeson].
That would have been weird, particularly if it had been Connery. I'd keep wondering if Aslan was some distant cousin of Draco from
Dragonheart.
I reckon, though, that Lucille Ball as Scarlett O'Hara would have made
Gone With the Wind almost watchable.
That could have, conceivably, totally changed the landscape of entertainment today. If Lucille Ball had landed such a plum dramatic movie role, her career could have gone along a very different path.
I Love Lucy might never have been made. And when you consider how integral
I Love Lucy was to the early years of television, you gotta wonder just how anything would have turned out.
How would
Star Trek have fared if they had stuck with Jeffrey Hunter as Captain Pike rather than replacing him with Captain Kirk (who would have originally been played by Jack Lord but he & the producers couldn't agree to a financial deal)? Would Pike & Spock have eventually found their rhythm the way that Kirk & Spock did or would the show have died a quick death? How would future revivals have been affected by Hunter's death in 1969?
Would
Daredevil still have such a bad rep if Colin Farrell had played Daredevil while Ben Affleck had been Bullseye? Would Affleck & Jennifer Garner still gotten married?
What would have occupied the tabloid headlines for the last 5 years had Brad Pitt's
Mr. & Mrs. Smith co-star been Nicole Kidman and not Angelina Jolie?
Instead of Bill Murray, Sigourney Weaver, Rick Moranis, & Ernie Hudson;
Ghostbusters could have starred John Belushi, Julia Roberts, John Candy, & Eddie Murphy. John Belushi died. A not-yet-famous Julia Roberts lost out to the more bankable Sigourney Weaver. John Candy said he would only play Louis if he got to do it with a German accent. And, I think, Eddie Murphy wanted too much money.