The reason that Witherspoon turned the role down was to offer space to a newcomer that she really believed in. She tossed the role to Neve Campbell, who ended up starring as Prescott in all five installments of the Scream movie series.
Emily Blunt, Iron Man 2
Scarlett Johansson took the role of Black Widow in Iron Man 2 in a heartbeat. But, Emily Blunt was actually the first person that Marvel approached about regarding the project. Blunt turned the project down, claiming that the paycheck was too low for her time.
Blunt doesn't regret letting Scarlett Johansson take the role of Black Widow. She's even into working with Marvel in the future on different projects that might need another superheroine.
“I’m never regretful about not doing something; I didn’t do it at the time because it wasn’t the right time and I was contracted to do something else,” Blunt reported to Screen Rants after Iron Man 2 was released. “But Scarlett [Johansson] has done such a phenomenal job—she’s it! Like why even reminisce about what could've been? She’s it and she nailed it, and I love The Avengers, I really found it to be really good fun.”
Leonardo DiCaprio, Boogie Nights
DiCaprio was a hot (and we mean HOT) commodity early on in his career. After Paul Thomas Anderson spied DiCaprio acting in The Basketball Diaries, he knew that he needed to ask him to star in Boogie Nights. Unfortunately, DiCaprio had already signed the contract to play Jack in Titanic, but he did offer a potential solution to Anderson...