Generally you want to walk onto a movie set with your script as finished as possible. You definitely want flexibility - filmmaking and editing are processes of discovery - but you also want the road map as clear as possible. At the very least you want to walk on set with all of your major story beats and action scenes figured out, if only because when it comes to action you need to storyboard, prep FX, find locations, rehearse and all that good stuff.
Which is why Mission: Impossible 5 shutting down so director Christopher McQuarrie can figure out a new ending to his movie-in-progress is noteworthy. This isn't just a tweak; according to the Hollywood Reporter McQuarrie and an unnamed screenwriter are sitting down to completely overhaul the film's finale. Paramount is spinning this as a positive turn of events, despite just having moved the film up from Christmas to the summer.
"Chris, Tom [Cruise] and a third person wanted to take a minute to get from what they thought was a good place to a more perfect place," a rep told the Hollywood Reporter.
Note the "a third person" thing - this new writer is not being credited or paid. Is it Simon Pegg? I'm trying very hard to think who could have come out to London, where the film is shooting, to lend a hand. Pegg makes sense, but so do any number of screenwriter pals of McQuarrie. The secrecy is likely for guild reasons, and if the writer is being unpaid (it's not at all unusual for writers to come on to a movie and be uncredited, but unpaid is very unusual) then Pegg is, again, mostly likely your man.
It's easy to assume this is a disaster in the making, but I kind of like the ballsiness of the move. And it isn't like Paramount has failed by totally overhauling their endings before - this could be the next World War Z, where a troubled finale ends up meaning big box office.