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Marvel actually had to go back to the MPAA three times to lock in a PG-13. The sticking point was Coulson's death - the first cut had Loki's staff bursting clean through his chest, which was way too gory. Whedon dialled it right back to just a stab sound effect and Thor's reaction selling the horror of it, and honestly it hits harder for being left to your imagination.
You don't actually see the 'Marvel's The Avengers' title screen until roughly twelve minutes into the film, well after Loki's already caused chaos at the SHIELD facility. Small detail, but it lands as a proper 'this is it' moment - four years of setup movies finally paying off in one title card.
This was the movie that proved the whole 'shared universe' gamble had actually paid off. It became the first Marvel Studios release to break the billion dollar mark, overtaking The Dark Knight's record in the process and becoming the highest grossing comic book movie ever made at the time.
Thanks to how far motion capture had come by 2012, Ruffalo wasn't just voicing the Hulk in cutaways, he actually performed the majority of Hulk's physical movement himself in a mocap suit. It's the first time in a live action film that both sides of Banner were played by the same actor, and you can genuinely see Ruffalo's mannerisms in the big guy if you look closely.
Evans was never a fan of his Captain America costume in this one, particularly the helmet, since it had no chin strap and kept sliding around his head between takes. It became enough of a running joke that later MCU movies actually poke fun at the costume whenever the events of this film get brought up.
Renner wasn't happy that Hawkeye spent most of the film brainwashed by Loki instead of doing much actual archery. He was so frustrated with the lack of character work that he reportedly tried to have Barton killed off just to be done with it. He's since said he's glad it didn't happen, given how much more the character got to do in later films.
Watch the post credits scene closely and Chris Evans is the only Avenger not eating, and he's clearly holding his hand up against his jaw the whole time. That's because he'd already started filming Snowpiercer and refused to shave his beard, so the makeup team stuck him in a wig and a prosthetic chin and hoped nobody noticed the awkward hand placement covering the seams.
While Ruffalo's face got mapped onto the character, his build didn't quite match the Hulk's proportions. The CGI team based the physique on bodybuilder Steve Romm instead, then layered Ruffalo's facial performance and expressions on top of that frame.
That whole silent shawarma scene almost didn't exist. It wasn't in the original screenplay at all - the cast and crew shot it the day after the film's global premiere once Whedon decided the joke from earlier in the film deserved a payoff, and now it's basically the blueprint every MCU post credits scene since has tried to live up to.
Downey Jr apparently kept snacks hidden all over the lab set so he'd never go hungry between takes. In one scene where he offers Chris Evans some blueberries, that wasn't scripted at all, it was just Downey munching on his stash and deciding to share, and Whedon liked the moment enough to leave it in the final cut.
