According to a cast member named Simon Callow's confession in 2014, Roberts pulled out because Daniel Day-Lewis turned down his prospective role as Shakespeare. Paltrow ended up starring with Joseph Fiennes, who she had a history of working with. This combination of powers proved to be massively successful, netting the film seven Oscars, including Best Picture.
Tom Hanks, Jerry Maguire
Jerry Maguire was actually written with Tom Hanks in mind for the titular role, but the actor turned down Cameron Crowe for a part in the 1996 sports drama because he was directing his own film, That Thing You Do. Hanks believes that all in all, even though he was the intended actor for the film, Tom Cruise was the perfect choice for the role.
“I think you look at it now and it couldn’t be anybody other than Tom Cruise,” Stated Hanks in an interview with Access Hollywood. “It’s the way the movie’s operated. I don’t think anybody would look at that now and say, ‘That movie was not perfect.’”
Warren Beatty, Kill Bill
David Carradine ended up stealing the show with his portrayal of Bill in Kill Bill. But, he wasn't actually supposed to assume the role. Warren Beatty was supposed to assume the position, but his schedule prevented him from shooting the movie in China.