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Once upon a time, Avengers: Infinity War and the still untitled Avengers 4 were supposed to film simultaneously -- in fact, in Marvel's initial Phase 3 announcement, they we're described as Infinity War Part 1 and Part 2. A lot's changed since then. After all, when the Phase 3 lineup was unveiled in 2014, Captain Marvel was set to arrive in theaters this summer and Marvel's Inhumans was set for the fall. Obviously, none of that happened (sorry, Inhumans), and in fact, nowadays the powers that be at Marvel firmly insist that Infinity War and Avengers 4 aren't a two-parter, but two very distinct films and along the way, the production also shifted to a back-to-back shooting schedule.

So why did filmmakers Joe and Anthony Russo decide to split up production on their Avengers epics? In short, it was just too much to keep track of. Each Marvel film -- especially the Avengers team-ups -- are a massive undertaking with a lot to keep track of, and according to the Russos, they realized in pre-production that the technicalities of filming simultaneously were just too demanding.

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Image via Marvel Studios

Last night, Collider hosted an exclusive screening of Avengers: Infinity War with the Russos in attendance, and the filmmakers explained the thought process behind splitting up the film.

Anthony Russo explained,

"Because these movies are so big and so complex and there are some character who are involved in both, there was an idea that we would mix both movies and would cross-board them in a way. But as you get closer and deeper into production, the movies were so complex and hard to wrangle just for our own process and everybody’s process involved on a creative level, we needed to separate the two...They’re really different movies and we wanted to treat them as very different movies, separating them."

Of course, that doesn't mean production was easy. The filmmaker (who got pneumonia on set) continued,

"We did shoot them back-to-back. Literally, we finished shooting Infinity War and we rolled right into the next avengers movie. We were literally shooting for a straight year without stopping. It was a very intense, very difficult year, a very challenging year. It’s a good thing there’s two of us. So we did separate them just because they were so complex."

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Image via Marvel Studios

Asked when they realized they needed to make the split, Joe Russo added,

"We started having production meetings that were lasting like nine or ten hours, because you’d have to talk about one movie and you’d have to talk about the next movie. And once we were doing that, people were getting confused about what scene was in which film. It was draining everyone’s energy and resources. Each of these movies is as expensive as movies get, which means they’re very very advanced, complicated and difficult. It’s very difficult on the crew and it’s a lot of information to retain and we just felt like we had to separate them so we could focus."

However, the filmmakers also revealed that there were a few very select moments that they crossed over scenes from the two films and shot out of order. Asked if any actors had to shoot scenes from Avengers 4 while working on Infinity War, Anthony Russo explained,

"Only some of the actors, like maybe [Anthony] Mackie —like Joe said earlier, sometimes people can get confused because the movies are pretty complicated, and even though we did separate the two movies, there were a few instances where we were shooting out of order, mixing the films to a very light degree for very specific reasons, in a few instances. so it might have happened once or twice.

For more from our Q&A with the Russos, check out the links below and stay tuned for the full interview in the coming days.

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Image via Marvel Studios
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Image via Marvel Studios
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Image via Marvel