Loki, the adopted brother of Thor, teams-up with the Chitauri Army and uses the Tesseract's power to travel from Asgard to Midgard to plot the invasion of Earth and become a king. The director of the agency S.H.I.E.L.D., Nick Fury, sets in motion project Avengers, joining Tony Stark a.k.a. the Iron Man; Steve Rogers, a.k.a. Captain America; Bruce Banner, a.k.a. The Hulk; Thor; Natasha Romanoff, a.k.a. Black Widow; and Clint Barton, a.k.a. Hawkeye, to save the world from the powerful Loki and the alien invasion.
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Whedon's first pass at the script actually featured Ant-Man and the Wasp, the original founding Avengers in the comics, rather than Hawkeye and Black Widow. He'd even wanted Zooey Deschanel to play the Wasp, but the plan got swapped out in favour of characters Marvel already had set up on screen.
When Downey Jr first read the script he pushed hard for Tony to lead the opening sequence, telling Whedon 'I need to be in the opening sequence, Tony needs to drive this thing.' They actually tried filming it that way, but Downey himself admitted it didn't work, saying everyone else just ended up feeling like 'an arm of the octopus' around him.
The idea of a bigger villain pulling strings behind Loki actually came from Whedon himself. He suggested the Mad Titan Thanos as that hidden mastermind, got him written into the post credits scene, and had no idea at the time just how central that character would become to the entire franchise years later.
Hulk's iconic put down of Loki wasn't invented for the film. In the comics the Hulk's usual go-to insult is 'puny humans', so swapping it to 'puny god' for a literal god was a small tweak that turned into one of the most quoted lines in the entire MCU.
Whedon wrote a tight script overall but gave his cast room to improvise, and Downey Jr made the most of it. Lines like his dig at Thor, 'Doth mother know you wear-eth her drapes?', along with his closing 'let's just not come in tomorrow' at the end of the film, weren't in the original screenplay at all.
Taking over Bruce Banner from Edward Norton wasn't easy for Ruffalo, and he apparently spent the first few weeks of filming feeling seriously insecure about it. Both Whedon and Downey Jr had to keep reassuring him he was the right fit, with Ruffalo later saying Norton had essentially 'bequeathed' the part to him.
Kevin Feige deliberately tucked Nick Fury's Avengers Initiative cameo after the credits so it wouldn't overshadow the main film if the bigger universe plans never panned out. He was stunned by how fast it blew up anyway, with outlets covering it within days.
The famous Hermes statue at Grand Central Station was smashed to pieces during the Chitauri invasion in the first Avengers film. Look closely in this sequel and you'll notice it's been rebuilt, only now redesigned as a tribute to the first responders who helped out during that same Battle of New York.
The AC/DC track blasting as Tony makes his grand entrance at the Stark Expo is 'Shoot to Thrill,' and Marvel brought it back two years later for the moment Tony faces off with Loki in his tower during The Avengers.
