What exactly does that mean? Well, more often than not reshoots are used to beef up or sometimes add individual scenes to what is usually at least a somewhat complete narrative. In other words, there is a beginning, a middle, and an end, and the extra shooting is used to strengthen the connective tissue between those three.
But in the case of Avengers 4, things are apparently somewhat more fluid. Speaking on The Marvelists podcast, Ruffalo said that Avengers 4 is still going through changes, and the way the epic story ends is perhaps not quite set in stone:
“We’re doing reshoots starting in September, and then we’ll go back into the international tour, we’ll go on tour. Then you guys will get the second installment. Which we don’t even know what it’s gonna be yet. We’re not just doing reshoots; we’re going to finish the movie, which we really didn’t get to finish totally when we left it last year.”
Now, does this mean that half the film is being junked and redone, as was the case with Justice League, or that an entirely new ending is being conceived, as it was for, say, World War Z? Not quite. But Ruffalo admitted that the cast members don't know how the story plays out because directors Anthony and Joe Russo are still finding out themselves:
“I don’t even know that they really know exactly. Some of it is happening while we’re there. It’s pretty amazing. And we’ll shoot some stuff and a few days later come back and reshoot it 'cause we wanna take it in another direction. It’s a very living organism; even as we approach it being a locked picture, we’re still working on it.”
The term "living organism" is an interesting one, because it points to a different method of working than was deployed on either of the movies mentioned above. In the case of Justice League, Joss Whedon was brought in to rewrite and reshoot half a film after Zack Snyder left. With World War Z, multiple writers were tasked with coming up with an entirely new third act.
But in this instance, you have the same directors (the Russos) and possibly the same screenwriters (Christopher Markus and Stephen McFeely) who have been involved with the project since the start, along with producer Kevin Feige and largely the same creative team. It's the same core group of filmmakers presumably working to shape a film into (we hope) its best possible form instead of tossing away large chunks of the work they've already done in a desperate attempt to find something better.
Ruffalo added that the directors used tricks to keep the actors in the dark as much as possible while doing Avengers: Infinity War and the first round of shooting on Avengers 4:
"We were getting dummy scripts. That really blew my mind, because I was like, you have to really go out of your way to give everyone a script that isn't quite what's gonna happen, you know... some of us, I think, didn't even know what we were watching until we were actually watching it, finished."
The Russos' methodology seemed to work quite well, to say the least, on Avengers: Infinity War. But the stakes are even higher on Avengers 4, since the story will not only reverse the genocidal effects of the Thanos snap but alter the contours of the Marvel Cinematic Universe forever. Will the Russos find the resolution they're looking for? We have months to go before we know the answer to that.