Fun list from Cracked.com featuring both early concepts and originally considered actors for some truly memorable roles/characters:
Some of the early actor considerations, like Burt Ward (TV’s Robin) originally considered for the role made famous by Dustin Hoffman in The Graduate, was known to me. But John Travolta for Forrest Gump? Interesting! (I’ll repeat for the umpteenth time my favorite bit of original casting: Frank Sinatra as Dirty Harry! Reportedly a painful finger injury made Mr. Sinatra bow out of the film as he didn’t think he could handle firing that massive 357 Magnum. The role was offered to many other interesting names with various degrees of interest/disinterest, from Burt Lancaster to John Wayne to Robert Mitchum. Eventually Clint Eastwood took over the job and Dirty Harry the movie and character arguably became the most famous/iconic role he ever played!)
Also like the revelation that Krusty the Clown was originally supposed to be Homer Simpson himself! Makes sense as he did originally look an awful lot like Homer and the idea that Homer was secretly the character might have been an interesting concept for a few episodes.
Just goes to show that the creative process is rarely a very direct one. People can come up with concepts and ideas but they are refined and changed over time and circumstance…sometimes giving far, far better results than were originally conceived!