this video that shows the screen-test featuring Tom Selleck as Indiana Jones and Sean Young as Marion Ravenwood. Almost as exciting as the way the 25th Anniversary of Back to the Future yielded this video of Eric Stoltz as Marty McFly.
Of course as long as there's been movies, there's been close calls with actors nearly taking on iconic film roles that we can't imagine someone else playing. I call these "casting disasters" and covered it back at AMC. Here's a few examples: