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," starting with arguably the best sitcom ever ...

No. 5: Rosie O'Donnell as Elaine on "Seinfeld"

Back in the day, Rosie O'Donnell had been a aggressive yet good-natured performer who entertained audiences with her stand-up and on the screen. So it made sense when her agent called with a sitcom audition for a strong and funny female lead.

The character's name was "Elaine" for a new sitcom then known as "The Seinfeld Chronicles." This role seemed all the more appropriate as Rosie was quite familiar with Seinfeld's material. But, of course, the nod went to Julia Louis-Dreyfus who may be known as "Elaine" for all eternity.

But had O'Donnell gotten the role, how different the show would have seemed! This is especially true given Rosie's subsequent success and controversy as a TV personality. Looking back, this rejection was a significant determiner for a historic sitcom and a successful daytime career.

But our next case of alternate casting involves a much more established star missing out ...

No. 4: Johnny Depp as Jack in "Titanic"

Even harder to imagine are the biggest of the stars being rejected. But it happens.

Johnny Depp had to sit back and watch "Titanic" unfold to its box office glory with another actor at the helm: Leonardo DiCaprio. And you know darn well it must have been frustrating for Depp seeing all that success knowing he was turned down for the part. "It should have been me!"

But like any motivated and determined person, Depp just had to suck it up, realize that's how the business works, and say "on to the next one." This, of course, he has done multiple times over.

And while we're talking Depp, another of his misses is significant. Had he gotten the lead in "Mr. and Mrs. Smith," who knows if "Brangelina" would have happened. Who would the tabloids be writing about then?!

Our next snubbed actor was also upstaged by Leo ...

No. 3: Jim Carrey as Howard Hughes in "The Aviator"

Again, you'd think the power indicated from Jim Carrey's $20 million price tag would make him practically immune to competition. But he's not the only A-lister out there.

Plus when you're as versatile as Mr. Carrey, it's possible to pitch yourself for almost any role. So with the many "yeses" in his career, Carrey certainly has had his share of "nos."

Howard Hughes, the eccentric billionaire, might have been a fun role for Carrey, but the studios opted for the stylings of DiCaprio. (Hey DiCaprio, quit hogging all the parts!)

But at least here we get some redemption for Depp. Turns out that Mr. Carrey also auditioned for the part of "Willy Wonka" in the remake of the children's classic. So this time it's Depp's turn to say "naa na na, boo boo."

Our next spurned actor wants to know, do you feel lucky?

No. 2: Clint Eastwood as John Rambo in "First Blood"

Today this one seems a bit awkward, does it not? Clint Eastwood, the octogenarian director as Rambo?!

But, hey, let's get out of the present for a second. Remember that once upon a time he was a shirtless tough guy who hung with the best of 'em -- a tough cop, a legendary gunman; heck, he even had himself a pet orangutan in one flick! Seems fitting that he'd get his chance at war, right?

Well, Sylvester Stallone got Rambo, further defining his status in film. Just imagine -- all those Rambo references would have been forever altered with Eastwood behind the machine gun.

And then who knows if we would have been treated to another "Rambo" movie in this millennium? Probably not. Remember, Eastwood is now an octogenarian.

Last up, this next alternate casting would have resulted in one silly superhero, or two ...

No. 1: Ashton Kutcher as Batman or Superman

Can you see Ashton Kutcher kickin' butt and saving the day?

Yeah, neither could the casting directors for the superhero reboots "Batman Begins" and "Superman Returns." We've certainly seen funny men do drama before. But action? Seems kind of a stretch, so that's what makes these "what ifs" all the more fun.

For Superman, they ended up going with a lesser known, but more masculine, Brandon Routh. Indeed, thinking about Kutcher's fun-loving grin in the Superman get-up may have had many fans wondering if it was Halloween.

Casting him for Batman, though, would have been no laughing matter. The way the last two Batman films have succeeded, it would have been a shame to miscast this title role.

And as dark as the these films have been, it's no doubt that Kutcher's Batman would have looked more like Peter Pan at a Goth rock concert.