The Incredible Hulk
It might be impossible to imagine anyone but Mark Ruffalo, Edward Norton was previously considered for the role because of his history playing the angry green giant. Norton just didn't mesh well with the other Avengers on the cast list, so he was replaced with the creative and innovative vision Mark Ruffalo brought to the character.
'Til Death
Allison Stark was the daughter of 'Til Death's main couple, Eddie and Joy, but was cast and then recast...four times! After the first few switches, the episodes started mocking the actress swaps on-screen. Krysten Ritter played Ally for the first two seasons, but Laura Clery filled in the role in the next season, and Kate Micucci finishing the part in the last season.