A lot of people assume that big-budget movie actors have a lot of job security. After all, having a big star at the helm of a film can often ensure that it makes big bucks at the box office! It's in a studio's best interests to get prominent actors and actresses signed onto their films - and to keep them there.

However, it turns out that even superstars can get fired if they're not careful. A surprising number of movie productions have had to recast big roles following a firing or a "mutual parting of ways". Have you ever wondered about what could have been in the world of Hollywood blockbusters? Which stars lost their potential role of a lifetime? Here are 20 movie leads who were suddenly fired from major projects.

20 Chloe Grace Moretz - Bolt

Back in 2008, pop superstar Miley Cyrus bagged the lead (human) role in Disney's animated animal comedy Bolt. Originally, though, a different actress was going to lend her voice to Penny, the titular Pooch's devoted owner. Chloe Grace Moretz was all lined up to take the role, and actually recorded all of Penny's lines. However, Disney decided to partially replace her with Cyrus, one of their biggest teen stars at the time. They did at least keep Moretz on in the role of "young Penny".

19 Colin Firth - Paddington

Colin Firth is another actor who got the boot from an animated movie. Firth was originally slated to voice Paddington the bear in the 2014 movie of the same name. However, when he began to record his lines, both he and the film's producers realized that he wasn't right for the part. He left the production, with Ben Whishaw replacing him.

18 Natalie Portman - Romeo and Juliet

Turns out Natalie Portman was originally going to star as Juliet opposite Leonardo DiCaprio's Romeo! She was the first choice for Baz Luhrmann's 1996 movie. However, she was eventually replaced by Claire Danes when the film's producers realized that Portman was just too young for the role. To be fair, she was only 13 - while Leo was an actual adult aged 21. It would have just been creepy.

17 Jean-Claude Van Damme - Predator

Hollywood action superstar Jean Claude Van Damme's departure from Predator in the 1980s is shrouded in mystery. There are a lot of theories behind why he was fired. Was it because he hated the Predator suit, because of creative differences with his director, or something else? The only people who know are Van Damme and the rest of the Predator cast and crew. All we do know is that Van Damme was definitely fired from that movie.

16 Anne Hathaway - Knocked Up

In the years since the movie Knocked Up was released, lead actress Katherine Heigl has spoken in less than complimentary terms about the film. Awkwardly, she wasn't even supposed to star in the movie in the first place! Originally, Anne Hathaway was going to take on the lead role. However, she parted way with the movie because she didn't like the idea of real birth footage being used. Fair enough!

15 Christian Bale - American Psycho

Now, we know what you're thinking: Christian Bale did star in American Psycho! He wasn't fired from it, right? Well, actually, he was. The producers of the film hired and then fired Bale when they decided that they wanted Leonardo DiCaprio to be the star instead. However, DiCaprio's involvement didn't work out, so Bale was brought back in.

14 Samantha Morton - Her

The producers of Her left it a bit late when they decided to recast the pivotal role of Samantha. In fact, actress Samantha Morton had already recorded all of her lines for the film before it was decided that she wasn't right for the part. She was replaced by Scarlett Johansson in post-production, with some reshoots of the film being necessary.

13 Harvey Keitel - Apocalypse Now

The production of the movie Apocalypse Now is notorious for having run into all manner of problems. One of the main issues that led to filming being delayed was the firing of the film's lead actor, Harvey Keitel. It emerged that Keitel's interpretation of his character didn't match the director's vision. Martin Sheen was hired as his replacement - and, in a further twist of fate, actually suffered a heart attack during production. Talk about a cursed process!

12 James Purefoy - V for Vendetta

V for Vendetta's masked villain has become pretty iconic over the years. Although the character was eventually portrayed by Hugo Weaving, it was James Purefoy who originated the role. While he eventually left the movie due to creative differences with the director, some of his scenes did make it into the film as his identity was hidden by the mask. His lines were simply dubbed over by Weaving.

11 Megan Fox - Transformers 3

Megan Fox's firing from the Transformers series has become a pretty infamous Hollywood tale. It's pretty well-known that Fox had a major falling-out with the franchise's director, Michael Bay. After Fox referred to Bay as being like "Hitler", she was dismissed and replaced by Rosie Huntington-Whiteley. Bay then went onto claim that Stephen Spielberg forced him to fire Fox, although that accusation hasn't been confirmed.

10 Lindsay Lohan - The Other Side

The Other Side was supposed to be the movie that revitalised Lindsay Lohan's career. In the end, though, it was a production that just didn't work out for the actress. She was originally going to play the lead role of Max Mackenzie, she eventually parted ways with the project. It's been claimed that the movie's studio no longer believed that casting Lohan would bring them in enough money. Ouch!

9 Robert Downey Jr. - Gravity

He may be known around the world as Iron Man, but Robert Downey Jr. almost made waves in another major Hollywood role. Downey was slated to take on George Clooney's eventual role in the blockbuster Gravity. However, he left the project pretty early on in production. The actor has since claimed that he struggled with the technology involved in making the film.

8 Nicole Kidman - Panic Room

The reason behind Nicole Kidman's departure from the movie Panic Room is pretty sad, to be honest. During filming, it quickly became clear that Kidman wouldn't be able to fulfil the demands of the role because of an injury she'd picked up while shooting another movie. For her own health and safety, Kidman had to be replaced. Her role was eventually taken on by Jodie Foster.

7 Stuart Townsend - The Lord of the Rings

It's hard to imagine anyone but Viggo Mortensen in the role of Aragorn in the Lord of the Rings series. However, Stuart Townsend was almost the person who took on this iconic part! In fact, he was actually involved in filming for a couple of days before director Peter Jackson decided he was the wrong man for the job. Poor Stuart!

6 Annette Bening - Batman Returns

Michele Pfeiffer's portrayal of Catwoman is one of the most iconic versions of this well-known character. However, Pfeiffer almost didn't get her role in Batman Returns. Originally, her part had gone to Annette Bening. However, Bening became pregnant and had to drop out of the movie. Pfeiffer was brought in, much to her delight - it was apparently her dream role.

5 Eric Stoltz - Back To The Future

Fun fact: Michael J. Fox was always the director's top choice for the role of Marty McFly in Back To The Future. However, it initially looked like he wasn't going to be available for the filming of the movie due to prior commitments. For that reason, Eric Stoltz was cast as Marty. This quickly turned out to be a mistake, though - Stoltz just wasn't right for the part. He was axed, and Fox was brought in.

4 Judy Garland - Valley of the Dolls

Judy Garland's firing from Valley of the Dolls is pretty infamous. The iconic actress was apparently sacked after turning up to work inebriated on alcohol and other substances.  She was then replaced by Susan Hayward. However, it's been claimed that the film's director, Mark Robson, treated Garland pretty badly during her brief time on set. The firing was just one part of Garland's tragic downfall; she passed away two years later.

3 Ryan Gosling - The Lovely Bones

Ryan Gosling's dismissal from The Lovely Bones is another example of an actor and a director having different visions for a character. Gosling was slated to play Jack Salmon, a grieving father struggling with the tragic circumstances of his daughter's demise. Gosling believed that the character should be large in stature, and consequently put on weight for the role. However, Jackson disagreed with Gosling's interpretation of the character, and eventually replaced him with Mark Wahlberg.

2 Julianne Moore - Can You Ever Forgive Me?

Julianne Moore was fired from Can You Ever Forgive Me? for a pretty unique reason. According to Richard E Grant, the film's lead actor, Moore wanted to wear a fake nose and a fat suit to portray the character Lee Israel. However, the movie's director and producer were adamantly against this idea and decided that Moore wasn't the right fit for the role. She was replaced by Melissa McCarthy, who bagged an Academy Award nomination for her portrayal of Israel.

1 Richard Gere - The Lords of Flatbush

Ah, The Lords of Flatbush: the film that ignited the long-standing feud between Richard Gere and Sylvester Stallone. Apparently, Gere and Stallone clashed so badly on the set of this movie that the producers decided that one of them had to go. They simply couldn't work together. It was ultimately Gere who got the chop; he was replaced by Perry King.