The film notoriously saw half of all humanity wiped out – thanks to Brolin’s supervillain Thanos.
Among the deceased were a huge number of Marvel heroes; among them King T’Challa, Spider-Man, Scarlet Witch and Doctor Strange, as well as several of the Guardians of the Galaxy.
Now, Brolin has said that he had no idea who – and how many – would die until he was sat in a cinema watching the movie.
“I was literally sitting in the theater going, ‘Oh my God. Oh my God. Spider-Man just disintegrated. That’s not OK,’” he told USA Today.
“I looked at [Chris] Hemsworth [actor, Thor] and he looked at me. He smiled and was like, ‘Your career is over.’ “
He added: “The Russos [directors] were like, ‘You’re doing something that is kind of creating a sympathetic version of this guy.’ And I was like, ‘Is that bad?’
“They were like, ‘Let’s go further with it.’”
Fans have obviously been distraught at the movie’s death toll – which Brolin said makes for a job well done.
“On Instagram people are posting things like, ‘I hate you,’ ” he told the US publication. “That’s fantastic.
“I’m, like, the one guy who smiles when he sees that. It’s like, ‘It worked.’”
Brolin will be seen back in Thanos mode for Avengers 4 next Spring, which is rumoured to see the returns of many heroes he killed off. The way in which that happens remains a total mystery.
In the meantime, fans can return to the MCU with Ant-Man and the Wasp in July, set immediately before Infinity War; and with Captain Marvel next Spring, set in the 1990s.
Marvel movies confirmed for after that include new Spider-Man movie Far From Home.
Avengers 4 is out on April 26, 2019.