Hollywood sure is a fickle beast, where even once an actor is cast in a massive, exciting new project, you can't be 100% sure of their involvement until cameras actually start rolling.
There are countless reasons why an actor might end up departing a project, with creative differences and scheduling issues being the most common factors.
The latter has been an especially tricky point in recent months since the end of the SAG-AFTRA strikes, ensuring that actors have had to re-assess and re-jig their schedules, in some cases forcing them to depart enticing projects due to clashes with previously planned work.
It's always a bummer to see a talented actor exit an intriguing film and one which will likely benefit their career, but alas, it can't always be helped.
And at least in the case of these 10 departures, the majority have already been replaced with some inspired substitutes, even if their names might not carry quite the same level of sway.
And yet of course, fans simply won't be able to avoid imagining what each film would've been like if the original casting choice panned out...
10. Steven Yeun - Thunderbolts
It was announced last February that Steven Yeun had won a coveted role in the Marvel Cinematic Universe's upcoming ensemble blockbuster Thunderbolts, and though Marvel Studios kept Yeun's precise part under wraps, The Walking Dead creator Robert Kirkman revealed last November that Yeun would be playing The Sentry.
In the comics, The Sentry is an extremely powerful Superman-esque hero who comes with a major liability: his powers are accompanied by a dark entity called The Void, ensuring every heroic act he commits is counter-balanced by a horrific one.
Excited though Marvel fans were to see an actor of Yeun's caliber working alongside the likes of Florence Pugh, Sebastian Stan, David Harbour, and so on, the actor apologetically confirmed at the start of the year that strike-related scheduling conflicts caused him to drop out.
Mere weeks later, however, the talented Lewis Pullman - son of Bill - was announced as Yeun's replacement, and while not as buzzy a name as the Oscar-nominated Walking Dead star, he's certainly got the acting chops to pull off such a pivotal role.