Tom Selleck Was Originally Cast as Indiana Jones
When Steven Spielberg and George Lucas were casting for Raiders of the Lost Ark, Mr. Spielberg liked the idea of Harrison Ford for the role. However, since George Lucas had already used him for Star Wars and American Graffiti he wanted to look for somebody new. They began auditioning for the role and came across an up-and-coming actor by the name of Tom Selleck and chose him for the part instead. At the time, Tom Selleck was also working on a new CBS sitcom called Magnum P.I. and hoped to work on both projects simultaneously. However, CBS decided that shooting for Indiana Jones would interfere with his work on Magnum P.I. and refused to release him. (Tom Selleck said in a later interview that shooting for Magnum P.I. was delayed and did not actually begin until shooting for Raiders of the Lost Ark had finished)
After they found out that Tom Selleck wouldn’t be available, Steven Spielberg again suggested using Harrison Ford and George Lucas agreed. Less than 3 weeks before filming began, Harrison Ford was cast as Indiana Jones and is now forever connected to the iconic character.
However, in an alternate universe somewhere, Tom Selleck really did get the part and each Indiana Jones Halloween costume comes equipped with a fake mustache.
Sources and Additional Reading: http://en.wikipedia.org/wiki/Indiana_Jones