What if Indiana Jones had been played by Tom Selleck? (He almost did.) How about Courtney Love in "Moulin Rouge"? (She tried out.) What if Forrest Gump had been played by Gary Busey? (OK, so we made that one up.)

But, it's easy to forget that in the world of filmmaking, competition takes place and nothing is in stone. It's real life.

Take casting. Roles usually aren't "written for" anyone. Thus, all those big names have to come out of the woodwork and make their pitch. And at the end of the day, the role goes to victor. But what if the victor had been another?

Jack from "Titanic" didn't have to go to Leo and the role of Rambo didn't have to go to Sly. And can you imagine anybody but Michael J. Fox playing Marty McFly in "Back to Future"? Stop imagining because the role originally went to Eric Stoltz.

Who tried out for which roles? And how different would things if Leo and Sly didn't win? Today, we look at five "what-ifs."

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