I haven’t looked into how accurate this is, but it’s good for a laugh nonetheless. The folks at wildammo.com used Photoshop to find out what movie posters would look like if the the films went with their original casting ideas.
My favorites: O.J. Simpson as “The Terminator,” Matt Damon and Ben Affleck as the “Brokeback Mountain” lovers, Kevin Costner in Harrison Ford’s “Air Force One” (ugh, really?) and Arnold Schwarzenneger and Sylester Stalone in “Face Off.”