Like pretty much everything in the artistic world, casting isn’t an exact science. Often times if the casting department can’t lock down the director’s first choice, it’s up to them to find the best actor for role, which can be a tough job. But an even tougher job is when they have to come up with a replacement for a role that has already been established by another actor. It’s certainly not a desirable position for anyone in the entertainment biz to be in, but, as these examples will demonstrate, sometimes it can actually improve a production.

10. James Rhodes/War Machine (Iron Man 2)

Nobody was complaining about Terrence Howard’s performance as James “Rhodey” Rhodes is the first Iron Man movie. He actually did a pretty great job. Or at least he certainly seemed to think so, because when it came time for Iron Man 2, he wanted a pay increase so that his earnings would be more in line with what Robert Downey Jr. was making. Pretty ballsy considering he was still a supporting character, albeit in a larger role than he had in the first movie.

In Howard’s defense, he did make $8 million to play Rhodes in the first movie and was only offered $1 million to reprise the role in the sequel. Meanwhile Robert Downey Jr. got a rather sizable pay increase. But instead of trying to work something out with the embittered Howard, Marvel Studios gave the role to Don Cheadle, and, by doing so, actually improved the character. Cheadle’s organic rapport with Downey was immediately apparent on screen and, since Howard never suited up as Iron Man’s sidekick in the first movie, Cheadle can still make the claim that he’s the original War Machine in the Marvel Cinematic Universe.

9. The Emperor (Return of the Jedi)

When we’re first introduced to Darth Vader’s master, the Emperor, in The Empire Strikes Back, he’s played by Elaine Baker and voiced by Clive Revill. But then, when we see the Emperor again in Return of the Jedi and as Senator/Supreme Chancellor Palpatine in the prequels, he’s played by Ian McDiarmid.

Though most people would probably never notice the casting change, especially since the character often wears a lot of makeup and is shrouded by a dark cloak, McDiarmid did such a good job with the role that today it’s hard to imagine anyone else as the surreptitious Sith Lord. Also, because George Lucas loves to go back and alter his classics, McDiarmid was actually digitally inserted into the remastered version of Empire Strikes Back. So if you’ve never seen the un-remastered version, chances are you thought the Emperor was played by McDiarmid all along.

8. Aunt Viv (The Fresh Prince of Bel-Air)

After the third season of The Fresh Prince of Bel-Air, it was no secret that Janet Hubert (the original Aunt Viv) and Will Smith had a lot of friction between them. And since the show clearly wouldn’t work without it’s leading star, it was Hubert who was sent packing. Still, things managed to move along pretty smoothly once Daphne Maxwell Reid took on the part of Aunt Viv. She even gave little fourth-wall-breaking wink to the audience in her first appearance after the character Jazz noted that something seemed slightly different about her.

7. Becky Conner (Roseanne)

In the 90s sitcom Roseanne, Lecy Goranson originally played the role Becky, the oldest of the Conner children. But, after five seasons, she decided it was time to leave the show and Hollywood behind her so she could focus on academics.

The series hastily replaced her with Sarah Chalke (best known as Dr. Elliot Reid from Scrubs), who made her mark by frequently joking about the discordant difference between the two young actresses. Chalke was solid in the role, but it was that shrewd sense of humor about the whole situation that really cemented the recast.

6. Rachel Dawes (The Dark Knight)

In the Batman comics, Rachel Dawes is a pretty minor character. However she became an essential motivating factor in Christopher Nolan’s Dark Knight Trilogy. Batman Begins saw Katie Holmes cast in the role but word has it she was pretty unpopular on set and her controversial marriage to Tom Cruise probably only made matters worse. This is likely what led to her replacement by Maggie Gyllenhaal in The Dark Knight. Of course, Holmes insists that it was all her decision to depart The Dark Knight series so she could make the movie Mad Money with Diane Keaton and Queen Latifah but, considering that movie didn’t even recoup its production budget at the box office and The Dark Knight went on to gross more than $1 billion, you’d really have to question her judgement.

Thankfully, things worked out just fine for Christopher Nolan as many fans and critics would agree that Maggie Gyllenhaal did a better job with role and The Dark Knight went on to become the most acclaimed superhero movie of all time.

5. Jennifer (Back to the Future II)

In Back to the Future, Marty’s girlfriend, Jennifer Parker, was played by Claudia Wells, but she didn’t have a very big part. Understandable given the fact that the time travelling teen protagonist spent the majority of the movie 30 years in the past. But at the end of the movie, Jennifer shows up again and ends up flying into the future with Doc and Marty in the DeLorean. This set the scene for Back to the Future II, which introduced audiences to a new Jennifer played by 80s idol Elisabeth Shue.

The first sequel is where Jennifer gets the majority of her screen time in the trilogy so it’s not surprising that a lot of people still think that Shue was the one and only actress to play the role. Interestingly enough, Claudia Wells wasn’t even the first to play Jennifer. During Back to the Future’s famously rocky start to production there were actually numerous changes made to the cast, even after filming had begun. Michael J. Fox stepped in to replace Eric Stoltz after shooting had already been underway for over a month, and before Wells came on board, Jennifer was played by Melora Hardin — who you probably know better as Jan from the U.S. version of The Office.

4. Catwoman (Batman – 1960s)

Batman certainly hasn’t had any shortage of villains to deal with over the years, but aside from the Joker, few have exhibited as many enduring qualities as his sometimes friend, sometimes foe, Catwoman. But things must have gotten a little confusing for Adam West during the production of the 1960s Batman Tv series because the role of the feline fatale was portrayed by three different actresses.

In the first two seasons Catwoman was played by Julie Newmar. Next up was Lee Meriweather who took on the role for the movie version of the hit TV show. Finally, and perhaps most memorably, the role was handed to Eartha Kitt, who not only broke racial barriers by filling a role that, up until then, had only been played by white actresses, but also managed to bring a more serious and ruthless side to the character — a praiseworthy accomplishment considering how that show was pretty much the epitome of cheesy dialogue and cornball acting.

3. Dumbledore (Harry Potter and the Prisoner of Azkaban)

When Richard Harris passed away in 2002, it put the filmmakers behind the Harry Potter series in quite a jam because it was intended that Harris reprise his role as the head of Hogwarts for the next five films.

Michael Gambon certainly had some big shoes to fill considering how spot on Harris’ portrayal of wise wizard proved to be, but, after the Prisoner of Azkaban, he owned the role and soon won over all the skeptics who had rightly grown so attached to Harris’ performance.

Though Gambon didn’t quite have the gravitas that Harris had, he added a sort of hippy whimsy to the character that perhaps made him even more likeable. As Dumbledore, Gambon did a great job of exhibiting the sly and playful side of the character that is often referenced in the books — a true testament to his skills as an actor, especially considering that he never even bothered to read the series.

2. The Hulk (The Avengers)

After all starring in their own live action movies, The Avengers assembled Captain America, Iron Man, Thor, and the Hulk all on the same screen. All the actors returned to reprise the roles from their associated film franchises save for one notable exception — Edward Norton was replaced by Mark Ruffalo as Bruce Banner, the Hulk’s alter ego. Though he might not fit the comic book depiction of “puny Banner” as well as Norton did, Ruffalo does such an excellent job of capturing the characters anger, heroism and loneliness, that he more than makes up for that shortcoming.

The decision to recast the Hulk for The Avengers movie mostly stemmed from Norton wanting to work on other projects. Norton has always been an actor seeking variety in the roles he chooses and he went on to star in Moonrise Kingdom, The Grand Budapest Hotel, and Birdman (all of which coincided with the production of The Avengers). Marvel Studios president Kevin Feige also stated that the decision to go with another actor for the Hulk was rooted in the need to find someone who embodies the creativity and collaborative spirit of the other talented cast members. So we’re guessing he and Norton didn’t part on the best of terms.

1. Daario Naharis (Game of Thrones)

The casting directors for Game of Thrones have done an absolutely superb job of finding actors and actresses that perfectly fit the descriptions of the sprawling cast of characters from the books. So much so that it now seems almost impossible to imagine anyone else taking on one the roles of a major character.

That being said, there have been a few cast adjustments over the seasons. The Mountain was played by two other actors before Hafþór Júlíus Björnsson settled into the role, and Dean-Charles Chapman replaced Callum Wharry as Tommen Baratheon after season three. But by far the most noticeable casting change was when Michiel Huisman took over the role of Daario Naharis from Ed Skrein in season four.

At the time, it was reported that the reason for Skrien’s departure was so he could take over for Jason Statham in the The Transporter franchise. But in an interview he gave while promoting The Transporter Refueled, Skrien stated that it was politics that got in the way of him completing his run on Game of Thrones.

Despite a pretty jarring change to the character’s physical appearance that resulted from the recast, Huisman now embodies Daario Naharis to the extent that he’s become one of the actors we can’t imagine would ever be replaced.