The script was not completely finished when filming began, since the filmmakers were more focused on the story and the action, so the dialogue was mostly ad-libbed throughout filming. Director Jon Favreau acknowledged this made the film feel more natural. 

“I am Iron Man” was ad-libbed by Robert Downey Jr.. Producer Kevin Feige approved using it in the final cut of the film, and credits this with his decision to largely do away with secret identities in the Marvel Cinematic Universe. Only Spider-Man conceals his identity, while Thor’s alter ego, Donald Blake, is similarly not used.

Paul Bettany has never seen the film, and is unfamiliar with the plot. He said J.A.R.V.I.S. was the easiest job ever, and it was almost like a robbery, since he only worked for two hours, got paid a lot of money, then went on vacation with his wife (Jennifer Connelly, who would later voice the A.I. in Spider-Man: Homecoming (2017)). His role would, however, be expanded considerably in the later Marvel films, and even require his physical presence from Avengers: Age of Ultron (2015) on.

Roughly 450 separate pieces make up the Iron Man suit.

The sound used during a target lock-on in Iron Man’s Head Up Display (HUD) is the sound of the laser cannon firing in Space Invaders (1978) video game.

Robert Downey Jr. was paid $500,000 for the film.

When the film was under development, it was never intended to set up a larger cinematic universe. As such, the post-credits scene featuring Nick Fury (Samuel L. Jackson) recruiting Tony Stark for the Avengers Initiative was cut in primary releases. In the post-Avengers re-releases (specifically since Kevin Feige took over production powers after Avi Arad’s departure) restore the post-credits scene and is now available on all prints.

Comic writer Stan Lee appears at Tony Stark’s party playing the role of Hugh Hefner, accompanied by three blonde women. Lee later mentioned that it was his most fun cameo.

The Iron Man Mark I armor weighed ninety pounds.