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When Loki turns up in the Gobi Desert right at the start of the show, the shot is staged to echo Tony Stark's landing after escaping the Ten Rings in the very first Iron Man film. It quietly puts Loki in that same "I just survived something huge" headspace Tony was in back in 2008.
Before Robert Downey Jr. ever tried on the suit, Tom Cruise was circling the role back when Iron Man was stuck in development at New Line. Nicolas Cage, a lifelong comic nerd, was also floated at one point. Wild to think how differently the MCU could've kicked off.
Robert Downey Jr. didn't have to screen test given his resume, but he did anyway, and he showed up dressed exactly like Tony Stark would: full tuxedo. It clearly worked, because Marvel handed him the role for a modest $2.5 million, pocket change compared to what he'd later earn.
Terrence Howard, fresh off an Oscar nomination, was actually the first actor signed to the film and was paid more than anyone else in the cast to play James Rhodes. When he asked for a raise on the sequel, Marvel said no thanks and brought in Don Cheadle instead.
Filming started before the screenplay was actually finished. The crew leaned so hard on the story beats and action that most of the dialogue got worked out on the day, take after take, with Favreau happy to let it happen because it made everything feel less rehearsed.
Phil Coulson was written as such a minor character that he wasn't even given a surname to start with, just 'Agent'. Clark Gregg clicked so well with the rest of the cast during filming that the part kept getting bigger, eventually growing into a whole spin-off show.
Marvel was so confident the Mandarin would be the villain that Favreau announced it at San Diego Comic-Con back in 2006. Plans shifted, though, and by the time cameras rolled the antagonist duties had gone to Obadiah Stane instead, with the Mandarin held back for a later film.
Jeff Bridges is a famously prepared actor, so having no real script to work from freaked him out at first. He eventually made peace with it by telling himself he was just part of a very expensive student film, and once he reframed it that way, he said it turned out great.
The man behind JARVIS's voice has apparently never sat through Iron Man and doesn't really know what happens in it. He's described the job as the easiest paycheck of his career, just a couple of hours in the booth before heading off on holiday with his wife.
