Speaking of Harrison Ford, he almost never had a chance to perform either of his most closely identified characters: Indiana Jones in Raiders of the Lost Ark and Han Solo in the first Star Wars film were originally offered to Tom Selleck, who turned them both down.
This actually made sense at the time, believe it or not. The original draft of Star Wars: A New Hope imagined Han Solo as much older than Luke Skywalker, like a kind of "black sheep" uncle figure, which honestly could have turned confusing with all the other themes of familial connection in the film. By the time Raiders of the Lost Ark was being filmed, Ford had been in three of George Lucas' films (American Grafitti and the first two Star Wars movies) and Lucas worried about becoming known for re-using certain actors. He gave Selleck another shot, but it didn't work out—Selleck was already committed to his series Magnum P.I., and the network wouldn't let him take the film.
Losing out on Raiders was a particularly bitter pill for Selleck, who ended up cooling his heels in Hawaii during an actors' strike…while Indiana Jones and company were also on the island. "I go to Hawaii to start Magnum, the actors go on strike, and I had given my deposit to a landlady and I couldn't afford a security deposit. So I start working as a handyman, in Hawaii, with no job. And guess who comes to Hawaii to finish their movie? Raiders of the Lost Ark," sighed Selleck. "So I could've done them both."