Actors that were originally suppose to play a role in the movie...

[D_Corky Cocque]

James Remar was replaced by Michael Biehn (Corporal Hicks) in Aliens.

Frank Sinatra was initially offered the Dirty Harry role.

Harvey Keitel and Jennifer Jason Leigh were originally in Kubrick's Eyes Wide Shut and filmed their roles but then completely replaced when they proved unavailable to do reshoots. Keitel was replaced by Sydney Pollack. Jason Leigh had the role of the prostitute.

Jon Finch started filming Alien but fell ill and was replaced by John Hurt.

 

[elegant20]

Here's another interesting one:

In It's A Mad, Mad, Mad, Mad World, Stanley Laurel was invited to make a guest spot in the movie, but due to his obligations and his dedication to Oliver Hardy, that never happened at all. After all, his last movie with Oliver was Utopia. Ever since Oliver's death, Laurel never made a movie without him and had kept his promise since.

 

[Mr. Snakey]

Here's another interesting one:

In It's A Mad, Mad, Mad, Mad World, Stanley Laurel was invited to make a guest spot in the movie, but due to his obligations and his dedication to Oliver Hardy, that never happened at all. After all, his last movie with Oliver was Utopia. Ever since Oliver's death, Laurel never made a movie without him and had kept his promise since.

Thats a interesting fact. I didn't know that. That is a great movie. A classic.

 

[MASSIVEPKGO_CHUCK]

Well, Selleck would've done it(He only did a test screen afterall!), but the producers from Magnum P.I. had to be pricks and not let him go on this one thing and get out of his contract.
Weirdest thing is- Both Selleck and Ford don't have dissimilar acting styles, so picturing either one in Raiders would've been easy.

**Fanfare**

The O-riginal screen test of Tom Selleck and Sean Young.
http://www.youtube.com/watch?v=qSyHSgEbNng

 

[elegant20]

In the original True Grit with John Wayne, the role of Mattie was passed down to Karen Carpenter, Tuesday Weld, Sally Field, and Mia Farrow. Mia Farrow later confessed turning down the Mattie role was the biggest mistake of her entire career before they decided on Kim Darby.

The role of Lebeuf is very interesting. John Wayne and director Hathaway originally wanted Elvis in the role, but his manager, Parker, insisted that Elvis receive top billing over Wayne! Imagine that! So it never happened and the role was then passed on to Glen Campbell.

 

[B_stu.kay823]

Michael Gambon was replaced as James Bond by Sean Connery

John Travolta was supposed to play Han Solo

 

[elegant20]

Another actor, Corey Haim did auditioned a lot of interesting roles thoughout his career:

The Goonies role of Mouth was then suppose to be play by Corey Haim, but I think Donner thought he was wrong for the part. The part was then given to Corey Feldman

In Stand By Me, Corey Haim auditioned for the part of Chris Chambers. The part then went to River Phoenix.

The Robin role in Batman Forever which Haim auditioned for, but he was strung out on drugs and forgotten his lines that he lost the part to Chris O'Donnell.