Every so often, a character comes along that is so embodied by the actor that you could never imagine anyone else ever playing them.
Of course, we see character recast all the time - James Bond, Batman and Superman are ones that spring to mind - but some will be more difficult to replace.
We take a look at some of the iconic roles that no other actor would ever be able to play.
- Robert Downey Jr - Iron Man
Robert Downey Jr would perhaps not have been the first actor who sprung to mind when this role was being cast - Tom Cruise was attached to this franchise at one point.
However, the casting of Downey Jr has proved to be a great move. Not only has this role re-established Downey Jr as a Hollywood heavyweight, but it is a role that allows the actor to truly ooze confidence, charm, and charisma.
The character of Tony Stark really does allow Downey Jr to strut his stuff, and he has become a major big screen hero in recent years.
The Iron Man franchise has enjoyed huge success since it hit the big screen back in 2008, with the third instalment grossing over $1 billion at the box office.
As well as being a great solo actor, Downey Jr shows that he also works well as a team with Avengers Assemble under his belt. While the future of solo Iron Man films is still uncertain, but no one could step into these really large shoes.
- Johnny Depp - Jack Sparrow
One of the most iconic characters to hit the big screen in the last decade or so is Jack Sparrow... a role that really elevated the career of Johnny Depp.
Up until this point, we were not use to seeing Depp in such mainstream roles, but Pirates of the Caribbean was a huge risk for Disney - pirate movies were not known for being major box office successes.
Depp strutted his stuff as Jack Sparrow for the first time back in 2003 in Curse of the Black Pearl, and it was another beautifully eccentric performance from the actor.
The moment he appeared on screen, film fans fell in love with this character, and it remains Depp's biggest and most famous role.
Since the release of Curse of the Black Pearl, three more Pirates of the Caribbean films have hit the big screen and been box office smashes.
A fifth film is in the pipeline, and you really couldn't imagine this franchise without the central performance from Depp.
- Harrison Ford - Indiana Jones
Indiana Jones remains one of Harrison Ford's greatest roles - along with Han Solo - and is one of the most iconic action heroes of all time.
We were introduced to Jones for the first time back in 1981 in Raiders of the Lost Ark, and the archaeologist and adventurer became an instant hit.
After Temple of Doom and The Last Crusade, it was almost twenty years until Indiana Jones returned with Kingdom of the Crystal Skull.
Jones/Ford may be a little older, and yet he still has that swagger that we have all come to love.
Shia LaBeouf was an actor that was tipped to be taking over the central role in the franchise, but Jones and Ford really do have to be the heart of these movies or they just do not work.
- Sigourney Weaver - Ripley
Ripley is one of the most iconic female film characters for all time, and was the film that really launched the career of Sigourney Weaver.
Released back in 1979, Alien is one the greatest sci-fi films of all time, and Ridley Scott's most iconic film.
There are so few female fronted franchises that can truly be considered as iconic, but it was Weaver's terrific central performance that really helped elevate Alien and this franchise.
Ripley's strength and fighting spirit is something that makes this character so memorable - this really is the character that all other great action female characters are measured against.
Rather than continue the Alien story or remake the films Scott made a prequel to Alien - a great way to get around having to recast Ripley.
For me, she remains one of the greatest female characters, and a recast would only result in a new actress doing an impersonation of what has gone before. I am delighted that this movie has never been remade.
- Bruce Willis - John McClane
We have already looked at one action icon, but now we are going to take a look at another in John McClane.
Die Hard hit the big screen for the first time back in 1988 and it was fast-paced movie that really was a lot of fun.
Die Hard really did help propel the career of Willis, and one of the best action franchises was born.
However, in recent years, we have seen the franchise begin to fade - the last film A Good Day to Die Hard has been the poorest film in the series so far.
John McClane is a character that well and truly belongs to Willis, and I would rather see this franchise end than see someone else come in and try to put their stamp on this terrific character.
- Tom Hiddleston - Loki
I suppose when you cast a supporting character in a new franchise you don't expect them to become the most popular... that is exactly what has happened with Loki.
Tom Hiddleston landed the role of Loki in Thor back in 2011, which he reprised in Avengers Assemble in 2012 and Thor: The Dark World in 2013.
Loki is a role that Hiddleston has really made his own, as he has become one of Marvel's best villains: and yet he is a villain that you cannot help but love.
The role of Loki has made Hiddleston a star and fans are calling for the character to get his own stand-alone film.
We will not be seeing Loki in the Avengers: Age of Ultron, but there is a third Thor film in the pipeline.
Of all of the Marvel roles, Loki - along with Iron Man - are the two that have made the biggest impact and the actors simply cannot be replaced.
Other roles that you will struggle to recast include Hugh Jackman as Wolverine, Daniel Radcliffe as Harry Potter and Heath Ledger as The Joker.