Katie Holmes was the first choice to play Buffy the Vampire Slayer.
Rosie O'Donnell auditioned for Elaine on Seinfeld.
Gene Hackman was rejected for the part of Mike Brady.
Sandra Bullock didn't have the right look for Baywatch.
Lara Flynn Boyle was considered to play Carrie Bradshaw on Sex and the City.
This is only a small sample. The shows are listed alphabetically, so this makes a very easy-to-use research book. You'll be surprised what you find out.