Stuart Townsend was originally cast to be Aragorn in "Lord of the Rings". Peter Jackson decided after a few days that he was all wrong for it and replaced him with Viggo Mortensen.
Burt Lancaster supposedly turned down a long list of famous rules, including "Ben Hur" (Charlton Heston), Patton (George C. Scott), and Dirty Harry (Clint Eastwood).
Brad pit was considered for the role of neo in the matrix but Keanu reeves eventually got the role.
will smith was also considered for the role of neo but turned it down and signed on to "wild wild west." he really should have gone with "the matrix" instead.
Will Smith is on the record as saying that he's glad he didn't get "The Matrix". He felt he would have screwed up the role. But that might just be him rationalizing it after the fact. It's also easy for him to say, since he's become a megastar anyway.
Tom Selleck was originally slated to plat Indianna Jones in Raiders of the lost ark, but was at that time still hooked with Magnum PI. Harrison Ford stepped in to take over.
Ironically, production on the first season of Magnum hadn't started yet at the time, but was scheduled to. It was delayed for so long that Selleck actually could have filmed Raiders before Magnum started taping, but no one knew that at the time.