Emily Blunt - Natasha Romanoff/Black Widow
Marvel's first choice to play the Black Widow in Iron Man 2 (and, feasibly, all other Marvel movies into perpetuity), Emily Blunt turned the role down due to bad timing. Then, again, Marvel came calling when it was looking to cast Chris Evans' love interest in Captain America. That role, she said, wasn't very good.
Source: Getty Images/Disney Studios