Robert Downey Jr. kept food hidden all over the lab set, and apparently nobody could find where it was, so they just let him continue doing it. In the movie, that's his actual food he's offering, and when he was eating, it wasn't scripted, he was just hungry.
There were very few times that everyone was in town at once, but on one night when they were, Chris Evans sent them all a text message reading "Assemble" (the tagline to the movie), prompting a night out on the town. Clark Gregg has stated that this is his favorite text message that he has ever received.
Joss Whedon explained that two of the founding Avengers members, Ant-Man and Wasp, were cut from the script because the film had too many characters. Also, the screenwriters didn't want Ant-Man in the Marvel Cinematic Universe until his movie was released.
Clint Barton (Hawkeye) is an ambidextrous archer (although Jeremy Renner is left-handed). He is seen shooting right-handed in Thor (2011) and both left-handed and right-handed in this film.
To prepare for the role of Agent Clint Barton (Hawkeye), Jeremy Renner was trained by Olympic archers. Director Joss Whedon later stated that he never gets tired of watching Hawkeye shoot arrows while looking the other way.