Hugh Jackman - X-Men:
It wasn't easy casting the role of Wolverine, first offered to Russell Crowe, who passed -- and Dougray Scott got the part. But scheduling conflicts with "Mission: Impossible 2" forced Scott to drop out, so Singer turned to Hugh Jackman, who joined three weeks into production. Six films later, Jackman's a superstar. Scott, meanwhile, is filming something called "The Vatican Tapes."
Michael J. Fox - Back to the Future:
Four weeks into production, Robert Zemeckis realized Eric Stoltz just wasn't working out as Marty McFly. Production halted until Michael J. Fox's schedule opened up -- and they started back at square one. "Back to the Future II" would make more moves, replacing the actors who played Marty's father and girlfriend.
Nicole Kidman - The Paperboy:
Sofia Vergara was set for the dramatic turn, but scheduling conflicts forced her to drop out of the Lee Daniels film. Nicole Kidman swooped in and won raves, getting Golden Globe and Screen Actor's Guild nominations. And let's not forgot the honor of getting to pee on Zac Efron.
Mariah Carey - Precious:
It's hard to see how replacing an Oscar-winning actress like Helen Mirren with singer Mariah Carey can be an improvement, but after Mirren left to do another project, Lee Daniels made that call. Carey impressed critics in a nearly unrecognizable role.
Don Cheadle - Iron Man 2:
Many pointed to Robert Downey Jr. for the success of "Iron Man," but Terrence Howard wanted some love, too. Howard demanded a pay raise for the sequel, but Marvel refused to budge and brought Don Cheadle in to play Col. James Rhodes, aka War Machine. Fans didn't seem to miss Howard, as "Iron Man 2" outgrossed its predecessor globally.
Harrison Ford - Raiders of the Lost Ark:
Indiana Jones nearly had an iconic mustache to go along with the fedora and whip. Tom Selleck was the first pick for the globe-trotting archeologist, but when he couldn't get out of his "Magnum P.I." contract, Spielberg got to go with his preferred choice, Harrison Ford.
Margaret Hamilton - Wizard of Oz:
According to Gale Sondergaard, the Wicked Witch of the West wasn't supposed to be an "ugly hag." When the decision was made to change up the look of the Witch, Sondergaard dropped out. Margaret Hamilton would be cast -- and effectively scared the pants off kids for years to come.
Martin Sheen - Apocalypse Now:
Francis Ford Coppola wanted Sheen for the role of Captain Willard, but he wasn't available. Harvey Keitel played the part for about a week before Coppola decided he needed to be replaced, and magically Sheen was now open to do it. Just one of the many wrinkles of what would be a legendary problematic shoot.
Paul Dano - There Will Be Blood:
The opportunity to work with Daniel Day-Lewis was a little too much for Kel O'Neill to handle, as the actor was intimidated by Day-Lewis' style of method acting and dropped out. Paul Dano came in to play the corrupt priest Eli and go toe-to-toe with Day-Lewis.
Arnold Schwarzenegger - The Terminator:
Arnold Schwarzenegger's silent and intimidating performance made the Terminator iconic. James Cameron, however, originally considered having football-star-turned-actor O.J. Simpson star, but decided he was too nice to play the oil-running killing machine. How ironic.
Kim Basinger - Batman:
"Blade Runner" star Sean Young was set as Vicki Vale in Tim Burton's "Batman." However, after breaking her arm falling off a horse during rehearsal, she had to be replaced with Kim Basinger, who reaped all the benefits from the blockbuster. Adding insult to injury, the horse-riding scene was cut.
Michelle Pfeiffer - Batman Returns:
Pfeiffer's sex-symbol performance as Catwoman in Burton's Batman sequel almost never came to be. Annette Bening was supposed to don the skin-tight suit, but got pregnant, and left the film.
Christian Bale - American Psycho:
Christian Bale was originally cast as Patrick Bateman, but was replaced with Leonardo DiCaprio. Then "Titanic" broke records and DiCaprio bolted from the indie. After a couple other flirtations, the role reverted back to Bale, which would eventually help him land the part of Bruce Wayne/Batman.
Viggo Mortensen - Lord of the Rings:
Four days into the shooting of Peter Jackson's "The Lord of the Rings" trilogy, Stuart Townsend was booted from the role of Aargorn after it was decided he was too young. In what may be the best backhanded compliment ever, the -- ahem -- somewhat older Viggo Mortensen came in to fill Townsend's shoes.
Clint Eastwood - Dirty Harry:
Old Blue Eyes, Frank Sinatra, nearly played tough cop Harry Callahan in "Dirty Harry," but injured his hand and couldn't take it on. Clint Eastwood must have felt pretty lucky after he came on in relief.
Eddie Murphy - Beverly Hills Cop:
"Beverly Hills Cop" was going to be a straightforward action film with Sylvester Stallone, then Paramount decided to flip the switch and make it a comedy with Eddie Murphy. The result was one of the most successful comedy franchises ever, with a possible fourth installment on the way.