I love this kind of Hollywood trivia. There's actually a whole website dedicated to it:
notstarring.com -- movie trivia
My favorite "what-might-have-beens":
George Raft. He was best known for his roles in old Hollywood gangster movies (including the original 1932 Scarface). He turned down the leads in both
Casablanca and
The Maltese Falcon. If he hadn't turned those roles down, he might have been as big a name has Humphrey Bogart, and Humphrey Bogart might not have been Humphrey Bogart.
Lucille Ball was one of the actresses considered for the role of Scarlett in
Gone with the Wind. Can you imagine Lucy Ricardo as Scarlett? We all know Lucille Ball as the queen of comedy and slapstick, but in the 1930s, it seems she was being groomed to be a Hollywood movie starlet like Ginger Rogers or Katherine Hepburn.
Ronald Reagan. He was considered for the role of Mr. Braddock in
The Graduate.
Gangs of New York. I enjoyed this film, but the one thing that almost ruined it for me was the casting of Cameron Diaz as Jenny. Worst casting ever. It's interesting to see all the actresses that were considered for that part. Because Scorsese had the movie in development for decades, the actresses considered varied widely. Very, very widely. For example, both Bridget Fonda and Jane Fonda. Both Arquette sisters. Everyone from Natalie Portman to Goldie Hawn.
Forrest Gump. Dave Chappelle, Ice Cube and David Alan Grier all turned down the role of Bubba. Tupac is said to have auditioned for the role. Nicole Kidman and Demi Moore both passed on Jenny. Bill Murray was considered for Forrest. Chevy Chase and John Travolta both turned down Forrest.
Speed. The role of Annie was originally meant for Ellen DeGeneres. Later on, Kim Basinger, Halle Berry and Alyssa Milano of all people, turned the role down. If they hadn't, we might not have the Sandra Bullock we have today.
More recently:
Silver Linings Playbook. Anne Hathaway turned the role of Tiffany down because of scheduling conflicts. (Good thing she had Les Mis for getting an Oscar.) Angelina Jolie was gunning hard for the part but Jennifer Lawrence swooped in at the last minute and beat her out, as well as Rachel McAdams, Olivia Wilde, Elizabeth Banks, Rooney Mara and Kirsten Dunst.
I could do this forever.