Casting directors play a thankless role in making movies, one most of us will never really, truly appreciate, unless the movie sucks. You’ve read the names Bonnie Timmerman or Ellen Lewis in the end credits a million times, but probably never stopped to wonder why they get such high billing right after the director.

First choices are not always best choices. As we approach the extreme boundaries of suspension of disbelief, the right actor can provide a dodgy film the gravitas it needed — or it can kick the movie right off the cliff of believability.

Directors lobby for stars to appear in their films, but that doesn’t mean that the actor in question fits the best in the role simply because they have the biggest star-power in the industry that particular month. In many cases, they are probably the worst, either too recognizable a name brand or simply the wrong persona. Why does Michael J. Fox work and Eric Stoltz doesn’t, though both are elite actors on the same level? Who knows, but sometimes a character works because of an actor, and sometimes the writing and actor are hopelessly incompatible no matter the tinkering.

Sometimes, the most tantalizing casting coups (George Clooney in Batman and Robin) turns sour, while the weirdest choices (the glittery, pouting emo vampire from Twilight in The Batman) cracks the code. The what-ifs scenarios are far weirder.

Stunt Casting

The Jerk Starring Steve Martin
Universal Pictures 

Based on his surreal portrayal of lovable moron Navin Johnson in The Jerk, Stanley Kubrick actually considered Steve Martin for his upcoming film Eyes Wide Shut. In the 80s, Martin’s brand of comedy was radically different from the old-school comics, and the bizarre mix of sincerity and slapstick intrigued the director, always on the lookout for a unique personality to plug into his films. "I know that his idea for it in those days was always as a sex comedy, but with a wild and somber streak running through it," Kubrick colleague and biographer Michael Herr once wrote.

He did manage to finagle comedian Peter Sellers into somewhat serious roles, but Sellers playing multiple roles in those black comedies was part of the joke. But the guy who croons about King Tut and played ukulele with arrows sticking out of his head was a bridge too far even for Kubrick to pull off. He thought better, delaying the whole thing for years until finally landing on dream actor, Tom Cruise, for a more serious portrayal of the crucial character of Dr. Bill Harford.

Related: 10 Roles That Were Turned Down That Would Have Changed Hollywood

Richard Dreyfuss excelled in supporting and snide, comedic roles like Jaws and Goodbye Girl, so to cast him as a globetrotting super spy who could mow down dozens of elite Martian special forces doesn’t quite pass the smell test. That’s almost what happened in the run-up to the 1990 sci-fi blockbuster Total Recall. Dreyfuss has the acting chops most others can only dream of. Comedic relief? Of course. Drama? Naturally. His stint in Close Encounters of the Third Kind proved his range, but nobody would be afraid of Dreyfuss with or without a submachine gun and holographic wristwatch. Dreyfuss is the ultimate every man, the one guy we can all identify with. Arnold Schwarzenegger, on the other hand, was born to play an amnesiac Martian double agent deep undercover as a construction worker who stabs evil scientists in the head and kung fu fights Sharon Stone armed with nothing but an endless supply of quips.

Adventures in Typecasting

Under Siege 2 - Warner Bros. Steven Seagal
Warner Bros.

The Schwarzenegger-Dreyfuss situation is pertinent to movie-making. When you launch a stand-alone film, you probably want a tent-pole actor to publicize it. When you relaunch an iconic IP you probably want a fresh slate, an actor no one really knows very well. The idea of James Bond sliding down a pole or dancing is pretty stupid, but Batman actor Adam West was a huge star with the name power to attract audiences in 1969, and after three years in the cowl and cape, producers knew he was willing to spout the corniest puns Bond writers could come up with a straight face.

One thing nagged at West: his ability to portray a British aristocrat. Sean Connery was Scottish, but the casting still fit. West's American accent didn't work. Roger Moore, another TV actor, landed the role instead, but only after West rejected it, according to friend and Batman co-star Burt Ward. His love of gadgets was a big plus, but his laid-back, non-violent persona was never going to jibe with the lusty, drunken secret assassin who killed his enemies in increasingly creative methods. We’re getting Dick Van Dyke vibes just imagining him trying to do a posh British brogue. Ward had his own regret, admitting he had to turn down a little film called The Graduate at the behest of his bosses at ABC.

Related: Every Major Franchise Role Sean Connery Turned Down, Explained

On the Batman theme, the rebooted Batman franchise in 1989 might have turned out far stupider if producers had their way and got their first choice to play Bruce Wayne. Steven Seagal was optimistically "floated" to play Wayne’s pointy-eared alter ego, writer Sam Hamm told SyFy. "He had just kind of appeared on the scene, people thought holy cow, this guy's badass. He could be Batman." Luckily, someone at Warner Brothers thought better and put the kibosh on Seagal. Tim Burton cast his friend Michael Keaton, which itself went against the actor's public image as goofy comedian. Considering how little mobility the costume afforded, all that martial arts experience would have been wasted anyway.

"Yippee Ki-Yay, Jack"

Take Me Out to the Ball Game
Metro-Goldwyn-Mayer

For better or worse, an action film is usually an extension of the lead actor, meticulously crafted to fit his or her personality and sense of humor. Cast a new actor, and you effectively have an entirely new film. That's scary, but opens up a lot of exciting new possibilities.

Die Hard (the novel source material) is actually based on a character that Frank Sinatra had played about 20 years earlier. Naturally, the crooner was the first choice for the role, reprising his character. In fact, he had to have first dibs at the big-screen adaptation because of his contract. A lot changed in the action genre in the decades intervening. Sinatra smartly turned it down. The thought of a 70-year old man in a homburg leaping off a skyscraper with a fire hose wrapped around his waist didn’t quite appeal to the old-school actor. And, it goes without mentioning, that Sinatra was probably not ever going to scream obscenities, effectively robbing the film of one the best lines in any action movie ever made. Die Hard is Bruce Willis.

To contrast the earlier example, Schwarzenegger was nearly in The Princess Bride, playing Fezzik in the role that went to Andre the Giant. Writer William Goldman had verbally given his promise to the small-time actor years prior, but by the time the movie entered the production stage in 1985, the Austrian bodybuilder had become a megastar, and was way too expensive. This turned out for the best, as Schwarzenegger’s brutal, energetic presence would have drastically altered the character, as compared to Andre the Giant’s lumbering, non-threatening delivery that transformed the role into a less-scary figure. These examples demonstrate why big-name actors making way for an unknown frequently provides a fresher, more interesting take on a character.