Ability over Appearance: Robert Downey Jr. was hired for having pretty much all of the traits that the character of Tony Stark had in the comics, with the exception of one: his height. Thankfully, there's the Scully Box to get around that problem.
Beam Me Up, Scotty!: As the first phase of the MCU culminated with The Avengers, many fans remember back to Nick Fury in The Stinger wanting to talk about "the Avengers Initiative", however the actual line is "I'm here to talk to you about the Avenger Initiative." (No "s".)
California Doubling: The Mojave Desert and Antelope Valley are used to double for Afghanistan. The rocks where Tony's military transport is ambushed are the Alabama Hills near Lone Pine.
Jon Favreau cast Robert Downey Jr. after seeing Kiss Kiss Bang Bang. He also believed that Downey's past made him an appropriate choice for the part and that he could not only make Stark a "likable asshole," but also portray an authentic emotional journey, once he had won over the audience.
Favreau cast Terrence Howard as Rhodey because he believed that he could play War Machine in a sequel. Obviously, that ended up not being the case.
Jeff Bridges grew a goatee and shaved his head, which he said was something he'd always wanted to do.
Peter Billingsley has a cameo as the lead Sector 16 technician (a.k.a. the guy Stane chides for not being able to reverse-engineer the Arc Reactor). He had his hair shaved to appear as male pattern baldness.
Averted with Robert Downey Jr.. He's in shape, but he's not the absolute beefcake that pretty much every other MCU lead would be in later movies.
The Foreign Subtitle: The film was subtitled Iron Man: El hombre de hierro in Latin American Spanish.
Harpo Does Something Funny: Apparently a good deal of the dialog was like this; not just Tony Stark's. It's reported that most of the script was a brief summary of what the actors needed to say, and from there they were allowed to pretty much improvise the finer details, which is why the dialogue feels a lot more naturalistic. This is how everyone discovered Robert Downey Jr. is a witty bastard, hence Iron Man's new characterization as a Deadpan Snarker. Jeff Bridges described the experience as if he were working on a $200 million college film.
The Other Darrin: Famously, Terrence Howard plays Rhodey in this film, to be replaced by Don Cheadle for the rest of the franchise. Less famously, Gerard Sanders portrays Howard Stark in a photo montage early in the film, to be replaced by John Slattery in the later films (except The First Avenger) and Dominic Cooper in the TV shows (and The First Avenger).
Playing Against Type: After being known mostly for his benevolent roles (a boyish hero in TRON, a father figure in Seabiscuit and White Squall...), Jeff Bridges turns it in as a truly vile villain.
The film had a lengthy production cycle that dates back to the early 1990's. Jeff Vintar and Stan Lee pitched a story for Fox that would have had M.O.D.O.K. as the main villain, and Quentin Tarantino was approached as the director. Due to having too much on their plate with X-Men and the studio's other Marvel movies (as well as Tom Rotham's rumored dislike of superheroes), Fox ended up selling the rights to New Line.
A new script was written for New Line Cinema, which featured a cameo from Nick Fury to set the character up for his own movie. Subsequent drafts also featured Howard Stark (who was still alive in this version) as War Machine, the movie's Big Bad. The execs also made bizarre demands and suggestions, such as saying that Iron Man shouldn't fly and should suit up by using a toaster. David Hayter did some work on the script, while both Joss Whedon and Nick Cassavetes were approached to direct. When production stalled for too long, the license lapsed and the rights returned to Marvel. Whedon would later go on to direct The Avengers and Avengers: Age of Ultron.
When the film was officially set up at Marvel Studios as the start of the Marvel Cinematic Universe, the Mandarin was originally set to be the main antagonist. Mark Millar convinced Jon Favreau to go with Iron Monger instead, arguing that the Mandarin was a poor tonal fit for an origin movie. Eventually, a Mandarin appeared in Iron Man 3, even if it turned out to be a fraud, and Aldrich Killian was given a similar role (and dragon tattoos), but the real Mandarin makes his debut in Shang-Chi and the Legend of the Ten Rings. The actual plot would have involved the Mandarin attempting to tunnel beneath Stark Industries to steal Tony's tech.
Before the Mandarin was cut from the script, Stane wasn't supposed to be a villain at all. Jeff Bridges had already been cast and had to be briefed on his character's new role during filming.
An early draft of the script revealed Tony Stark to be the creator of Otto Octavius's tentacles from Spider-Man 2.
Another draft had Crimson Dynamo as the villain, aspects of the character were incorporated into Whiplash in Iron Man 2 by virtue of making him a Russian named Vanko, like the original Dynamo.
According to Jeff Bridges, Stane was originally supposed to survive the final battle against Tony, with the heroes opening up the destroyed Iron Monger suit to find that there was no corpse inside. Presumably this would have poised him to return for future movies.
There was originally a detailed explanation of the suit's HUD, but test audiences tuned out too quickly to be able to make sense of it, and were actually more confused by the explanation, so it was all taken out.
Samuel L. Jackson recorded an alternate version of the now-famous post-credit stinger, which featured Nick Fury referencing "radioactive bug bites and assorted mutants" during his speech to Tony. It's unknown if this was meant to imply that the then-recent Marvel films from other studios like Sam Raimi's Spider-Man trilogy or Fox's X-Men movies existed in the same continuity as Iron Man, but the lines were presumably omitted due to legal reasons (though ironically Spider-Man entered the films directly later on, and the Disney/Fox deal opens the door for the X-Men to appear). This alternate scene sat on the cutting room floor for over a decade until it was released as an extra in a limited edition box set of the entire Infinity Saga.
Even the ending of the film was an ad lib! Originally the MCU would have proceeded like the comics, with "Iron Man" being passed off as Stark's bodyguard, but Robert Downey Jr. decided at the last minute to ad lib Stark throwing away the script and revealing the truth. Kevin Feige took a chance on making such a dramatic change to the canon, which encouraged their later experimentation and the creation of a more original franchise.
The Mark II's icing problem was originally not the Chekhov's Gun it is in the finished film. It was added during reshoots when the filmmakers decided they needed an extra beat in the final battle.
There were scenes shot where Tony was shown breaking parts of a washing machine while doing Abu's laundry, which he would've used to help create the Iron Man suit. This didn't make it into the film, nor was it released as a DVD extra, as Kevin Feige was utterly embarrased by it.
Louis Leterrier really wanted to direct this film, but he was ultimately asked to helm The Incredible Hulk instead, which he was more than happy to do.
Writing by the Seat of Your Pants: The script was not completely prepared when filming began, since the filmmakers were more focused on the story and the action, so the dialogue was mostly ad-libbed throughout filming and scenes were laid out to the actors in concept via outlines rather than scripted lines; Jon Favreau acknowledged this made the film feel more natural. Some scenes were shot with two cameras to capture lines improvised on the spot; Robert Downey Jr.. would ask for many takes of one scene since he wanted to try something new. Gwyneth Paltrow, on the other hand, had a difficult time trying to match Downey with a suitable line, as she never knew what he would say.
