When it comes to most classic films, you simply can't imagine anyone else but that actor playing the characters.
Tom Hanks as Forrest Gump, Harrison Ford as Han Solo or Keanu Reeves as Neo seem like those actors were in mind when the film was being thought up, yet those weren't the first choices for those roles.
Can you imagine how the following classic films would have turned out if they'd have cast...
14. Robin Williams as Jack Torrance (The Shining)
When casting for the crazed, axe-wielding Jack Torrance in The Shining, Stanley Kubrick considered both Robin Williams for the role.
After watching Williams' performance on Mork & Mindy, Kubrick thought the comedy actor was too psychotic to play the possessed, murderous writer.
You could imagine Robin Williams in the role, but it'd be a certainly make his character of Mrs Doubtfire a lot more sinister if his career went down the same path.
13. Sean Connery as Gandalf
You couldn't imagine anyone but Sir Ian Mckellen in the robes and beard as Gandalf The Grey, but initially Sean Connery was the man in the frame, with a multimillion dollar deal offered to the Scotsman.
Connery, who retired in 2003 after having an awful time filming The League of Extraordinary Gentleman, was offered $6 million dollars to play the wizard in the Lord Of The Rings trilogy.
Not only that, director Peter Jackson offered the Scotsman a 15 per cent stake in the franchise's box office profits, which means he could have netted nearly $450 million from his performance.
Still though, it would have been pretty silly seeing Sean Connery yell: "You shall not passhh!"
12. Matthew McConaughey as Jack Dawson (Titanic)
All the way back in 1997 when James Cameron was thinking of actors who could play Jack in Titanic, the studio heads at 20th Century Fox wanted Matthew McConaughey to star as the poor young artist who wins his ticket aboard the ill-fated liner.
Director Cameron insisted on Leonardo DiCaprio playing Jack, and got his way in the end.
McConaughey would go on to star as DiCaprio's chest-beating mentor in The Wolf of Wall Street.
11. Mickey Rourke as Axel Foley (Beverly Hills Cop)
In the '80s, Eddie Murphy was one of the hottest properties in Hollywood, before he donned a fatsuit in all of his films, and his biggest role was Axel Foley in Beverley Hills Cop, but imagine if the role had actually been played by... Mickey Rourke?
According to producer Jerry Bruckheimer, Mickey Rourke signed a $400,000 holding contract to play Axel Foley, but pre-production dragged on longer than the Sin City star was prepared to wait and he let the contract expire and went off to do another film.
The script then went on to Sylvester Stallone, who rewrote it to include himself, but departed when studio execs deemed his ideas "too expensive" for Paramount. They then turned to Eddie Murphy.
Imagine how different the Beverley Hills Cop films would be with Rourke or Stallone as Axel Foley (or Axel Corbretti as Stallone wanted the character to be named).
10. Frank Sinatra as Dirty Harry
It would seem like Dirty Harry was written for Clint Eastwood, a violent maverick San Francisco cop who isn't afraid to shoot big holes in criminals, but it was initially Ol' Blue Eyes, Frank Sinatra who was first in the frame to play the Smith & Wesson-wielding inspector.
Unfortunately for Sinatra (but fortunately for the future of film), he broke his wrist while filming the Manchurian Candidate and was unable to hold Harry Callahan's signature weapon and had to drop out.
John Wayne was then offered the role, but didn't want "Sinatra's rejections", so up stepped Clint Eastwood to ask punks if they felt lucky.
9. Christopher Walken as Han Solo
Harrison Ford almost didn't get his most famous role of all time as Star Wars: Episode VI - A New Hope director George Lucas auditioned a number of actors for the role of the arrogant Millennium Falcon pilot, including Al Pacino, Jack Nicholson, Kurt Russell, Steve Martin and Bill Murray.
But the most unusual name that came up as a potential to play Han was Christopher Walken.
Can you imagine Walken, with his trademark unusual speech pattern, saying to Obi Wan Kenobi: ”The Millennium Falcon made the Kessel Run in twelve parsecs…that fast enough for you, old man?” - exactly, you can't
But with the help of Saturday Night Live, here's Kevin Spacey doing his interpretation of Christopher Walken's audition for Han Solo... it's a hilarious and accurate look at what could (but should not) have been.
8. Vince Vaughn as Donnie Darko
Fresh from the success of Swingers and the not-so-successful Psycho remake, Vince Vaughn - who was in his early 30s - was in the frame to play the 16-year-old Donnie Darko.
It went so far as him being offered the part, but Vaughn, much more sensibly than anyone else, turned it down because he was too old.
Also, Mark Wahlberg was interested in playing the rabbit-haunted Donnie, but only if he could play the part with a lisp.
Fortunately sense prevailed and the role was give to 19-year- old Jake Gyllenhaal, and a cult classic was born.
7. Jack Nicholson as Michael Corleone (The Godfather)
The Godfather is regarded by many as the greatest film of all time, but how would it have gone down if Paramount got their wish and cast Jack Nicholson as the viscious Michael Corleone?
Paramount thought the movie was a bit of a gamble and wanted an established star to play the heir to the Corleone family, and offered the part to Jack Nicholson.
Nicholson turned it down, and has since explained his decision, stating: “I knew The Godfather was going to be a great film, but at that time I believed Indians should play roles written for Indians and Italians should do the same.
6. Tom Selleck as Indiana Jones
Harrison Ford is as much Indiana Jones as he is Han Solo, you simply couldn't imagine any other actor with the fedora and whip as Dr Henry Jones.
But co-writer George Lucas implored director Steven Spielberg to not use Ford, as he'd already been in American Graffiti and Star Wars: Episode VI - A New Hope.
George explained on the making-of featurette: "I said, 'Oh, Steven. He's been in two of my movies. I don't want him to be my Bobby De Niro.'" In relation to De Niro starring in many Scorsese films.
Spielberg let Lucas have his way and auditioned Tom Selleck, who they both thought was perfect for the part.
Unfortunately for Selleck, he'd already filmed the pilot for Magnum P.I. and couldn't get out of his contract with CBS, so the role went to Harrison Ford in the end.
We're given a glimpse of 'what could have been' in a screen test video, and unlike most of the entries on this list, it's not too dramatically different to the Indie that we know and love.
5. Robin Williams as Hagrid (Harry Potter)
When Warner Brothers set about making the Harry Potter book series into films, they were thinking of ideas over who could play Hagrid, the big-hearted gamekeeper.
Warner decided that a 'big name' was necessary so picked Robin Williams for the role, but JK Rowling, who had final approval of all of the cast decisions as part of her deal, refused to entertain the idea.
Hagrid ended up being played by Robbie Coltrane, while the mind boggles at what the Jumanji star would have done with the gentle giant.
4. Billy Murray as Batman
Back in 1989 when Warner Brothers were coming up with the idea for the first live-action Batman film since the 1960 version with Adam West, they decided that campy was the right direction to continue.
So much so, Bill Murray was considered for the role of the caped crusader in order to bring the right amount of campy humour to the cowl.
Things took a different turn whenTim Burton signed on as director and cast his Beetlejuice actor Michael Keaton to play the Dark Knight.
According to Murray in an interview with MTV:“I've heard that story. And God, I would have been an awesome Batman."
We had the George Clooney Batman with the Bat-nipples - could the Bill Murray Batman have been worse?
3. John Travolta as Forrest Gump
"Life is like a box of chocolates, you never know what you're gonna get."
What we almost got, in fact, was John Travolta playing Forrest.
It's virtually-impossible to imagine the star of Saturday Night Fever as our history-making hero, but Travolta was the original choice to play Forrest.
Travolta passed on the role and has since admitted it was a mistake.
A mistake for him maybe, not for the rest of the world, as Tom Hanks' portrayal as Forrest was just perfect.
2. O.J. Simpson as the Terminator
Possibly the most controversial non-casting on the list, sees OJ Simpson - famous for being an NFL running back, featuring alongside Leslie Nielsen in the Naked Gun comedies and that, er, court case - up for role of the emotionless killing robot The Terminator.
It all stemmed from an advert for Hertz rental cars starring O.J. which caught the attention of Orion Pictures head Mike Medavoy.
Medavoy explained: “At the time, O.J. Simpson had one of those commercials for Hertz where he jumped over a counter and ran to get a rental car. It was all of that athletic stuff, which I thought the Terminator should have.
But director James Cameron wasn't so keen on the idea, as he couldn't imagine the NFL all-star as a killer: “This was when everybody loved him, and ironically that was part of the problem—he was this likable, goofy, kind of innocent guy" explained Cameron.
At the time, Arnold Schwarzenegger was lined up to play Kyle Reese, but when Cameron met him, he thought Arnie would be a much better fit as a cold-hearted robot killer and decided that would play the T-800.
In retrospect, The Terminator wouldn't have stood the test of time so well if O.J. Simpson was the time-travelling, Sarah Connor-hunting murderbot.
1. Will Smith as Neo (The Matrix)
Keanu Reeves' emotionless portrayal as the computer hacker-turned-saviour of mankind was right on the nose.
But what could have been, had it not been for Will Smith's decision to play the lead in Wild Wild West, Neo could have been very, very different.
The Men In Black star explained: “You know,The Matrix is a difficult concept to pitch. In the pitch, I just didn’t see it. I watched Keanu’s performance – and very rarely do I say this – but I would have messed it up.
"I would have absolutely messed up The Matrix. At that point I wasn’t smart enough as an actor to let the movie be. Whereas Keanu was smart enough to just let it be. Let the movie and the director tell the story, and don’t try and perform every moment.”
Which is basically Will saying that he would have done his trademark overacting in each scene and it would have overtaken the deeper concept and storyline of the Matrix.
Still, if Will had have done that, it might not have been as successful and we would have been spared the terrible sequels.