'Avengers 4' and 'Avengers: Infinity War' were shot ''back-to-back'' for a ''straight year'' as separate movies so people wouldn't get confused about the continuity in the Marvel Cinematic Universe.

Directing duo Anthony and Joe Russo brought their 'Avengers: Infinity War' movie to the big screen in 2018 which saw multiple characters from the comic books movies join the established superhero squad, such as Doctor Strange, Spider-Man, the Guardians of the Galaxy and Black Panther.

Initially, the 'Infinity War' story was intended to be a two-parter but ultimately the Russo brothers decided to make two connected but separate films because they were just too many story threads and too many other MCU projects to keep track of.

Speaking to Collider, Anthony said: ''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.

''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.''

Joe realised the split was needed when the production meetings started lasting up to ''10 hours'' and he could see everyone was getting confused about what they were trying to achieve.

He explained: ''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 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.''

'Avengers 4' is due to hit cinemas in May 2019.