It's no surprise the events of Endgame are being kept top secret. Not only does the film wrap up this era of the Marvel Cinematic Universe after 21 films, but it picks up after the devastation of 2018's Avengers: Infinity War, in which the villainous Thanos successfully wiped out half of all life in the universe.
It's clearly going to take something huge to return from something so destructive, but beyond confirmation that Captain Marvel returns to Earth to assist the broken Avengers, what actually takes place in this three-hour movie remains a secret.
Perhaps the stars are just keeping mum on details they're unable to reveal - but speaking to Larson and Renner, there's a genuine feeling from both performers that they have no idea how things are about to pan out.
"It's a giant, giant world, so you're not quite sure exactly what's going on," says Renner. "You just trust the hands of the very capable directors (Anthony and Joe Russo)."
It was Larson, says Renner, who was thrown completely in the deep end; the actress had to shoot her scenes in Endgame before she even filmed her first solo outing as the character in Captain Marvel. "You had to come in running, didn't you," he says. "That must have been so tough."
"Because I shot this first, I didn't really know what I was stepping into, how much time had passed, whatever," says Larson. "I really could only work with the material that I had."
Given it was her first rodeo under the mantle of Captain Marvel, Larson says she was still exploring who the character of Carol Danvers was - as well as learning how to think, act and move like a superhero. "I was trying to get my sea legs and figure out even just how to stand," she says. "It was so funny - when you're all kind of standing around with all these superheroes, and everyone's like, 'well, who's gonna be the one that gets to cross their arms? And where do I put my arms? Do I have my hands in a fist?'
"It's just a different way of embodiment in these movies than with the films I have done previously," she says. "[I had to] think about a sense of ownership, and inner strength and this meeting of the minds that I think is a really powerful aspect of this film."
Given that the Avengers have now been teaming up for a decade - the original six members (Robert Downey jnr, Scarlett Johansson, Mark Ruffalo, Chris Hemsworth, Chris Evans and Jeremy Renner) even have matching tattoos - Larson faced being the new kid in school on set. But what could have been a daunting task was alleviated by her castmates, who welcomed her with open arms.
"I felt so lucky because these people have all been together for a really really long time and they're so welcoming and wonderful," she says.
"We had done this Marvel 10-year anniversary photo, that was my first kick-off with this, so I got to meet everybody," she says. "It was lovely to actually be around everybody, and at the reception for that photo, talk to everyone and ask for tips, because I felt like it created a good foundation; before then, I was on my own."
Directors Anthony and Joe Russo are two of the very few people in the world who actually know what's about to go down in Endgame. The brothers are determinedly tight-lipped on spoilers, and have been throughout the entire production - but it's not something they're enjoying.
"It's actually a real burden," says Anthony. "We work so hard to maintain the secrecy of the narrative, but we do that just because that's the way we want to experience it in the movie theatre. It's not an easy way to work; it's a very inefficient, difficult way to work."
Operating at such a high level of secrecy on a film brings obstacles at every corner, particularly when details are withheld from the performers - whole lines were reportedly redacted from the script, while the notoriously loose-lipped Tom Holland (Spider-Man) wasn't given a script at all.
"We end up having to do a lot more communicating on set with actors about what we're trying to achieve," says Anthony, "because they aren't getting the full vision of things in a screenplay. It's a tough way to work, but we think it's worth it in the long run for sure."
The brothers already pulled off one of the biggest shocks in pop-culture history with the end of Infinity War, killing off multiple beloved characters in the Marvel universe such as Black Panther, Spider-Man, Scarlet Witch and the entire Guardians of the Galaxy (apart from Karen Gillan's Nebula, and Rocket, the feisty raccoon voiced by Bradley Cooper). Audiences around the world were blown away by the film's brutal, emotional climax, and it's safe to say Endgame has more where that came from.
"We do like surprises in storytelling," says Joe. "It's clear from all of our prior work with Marvel, and this is an ending of 21 films and 11 franchises that are intertwined, and it requires a lot of thought and a lot of compelling moments.
"I think that ultimately people are in for a very cathartic experience because endings can be cathartic. But they can also be the most important part of the story."
That sense of catharsis is central to the Russo's film-making style. While there's action and spectacle, the fantastical elements of the Avengers films are always grounded in palpable human emotions. While these characters have extraordinary abilities, their conflicts and relationships are immediately recognisable to audience members.
"We grew up reading comic books in the late 80s when authors were starting to deconstruct traditional heroes and pull them into the real world and apply psychological realism to those characters - as much as you can with somebody in a superhero costume," says Joe.
"That's what's compelling to us, and what we've committed to in all of the movies that we've made, is really trying to be emotionally truthful and illustrate emotionally truthful behaviour."
And after four turns at the helm of Marvel movies, the Russos found their own emotions running high.
"Joe and I now have been working for seven years straight on Marvel movies, we've made four movies in that period, and that's a big chunk of our lives," says Anthony.
"It's a huge responsibility and a huge privilege to be able to write the final chapter in this book that so many amazing people have contributed to over the past more than 10 years. It's hard to imagine a more meaningful moment in a career than being in the place we are right now."
LOWDOWN:
Who: Brie Larson, Jeremy Renner, Anthony and Joe Russo
What: Avengers: Endgame
When: April 25
