Quentin Tarantino actually believes that Beatty made the right decision by backing out. He stated after the movie was released that he “was a little worried that Warren Beatty might look faintly ridiculous in this.”
Matt Damon, Brokeback Mountain
While it might be difficult to imagine a world where Ang Lee’s Brokeback Mountain stars anyone but Heath Ledger and Jake Gyllenhaal, when the project was in its beginning stages, Gus Van Sant approached a multitude of different actors. One of them was Matt Damon.
“Nobody wanted to do it,” Van Sant reported to IndieWire after receiving overwhelming "no's" from the individuals he asked. Damon, who had worked with Van Sant on Good Will Hunting, told the director flat out: “Gus, I did a gay movie (The Talented Mr. Ripley), then a cowboy movie (All the Pretty Horses). I can't follow it up with a gay-cowboy movie!”
Christina Applegate, Legally Blonde
Reese Witherspoon played the role of her career in Legally Blonde. But, originally Christina Applegate was originally asked to play Elle Woods. Applegate didn't want to get pigeonholed as an airhead blonde, so she decided to pass on the role.