• Robert Downey Jr. as Tony Stark / Iron Man: An industrialist, genius inventor, and playboy, he is CEO of Stark Industries and chief weapons manufacturer for the U.S. military. Director Jon Favreau felt that Downey's past made him an appropriate choice for the part and that the actor could not only make Stark a "likable asshole," but also portray an authentic emotional journey, once he had won over the audience. Favreau was also attracted to Downey because of his performance in Kiss Kiss Bang Bang. Downey frequently spoke with that film's director, Shane Black, about the script and dialogue in Iron Man. Downey had an office next to Favreau during pre-production, which allowed him greater involvement in the screenwriting process, especially when it came to adding humor to the film. Downey explained, "What I usually hate about these [superhero] movies [is] when suddenly the guy that you were digging turns into Dudley Do-Right, and then you're supposed to buy into all his 'Let's go do some good!' That Eliot Ness-in-a-cape-type thing. What was really important to me was to not have him change so much that he's unrecognizable. When someone used to be a schmuck and they're not anymore, hopefully they still have a sense of humor." To get into shape, Downey spent five days a week weight training and practiced martial arts, which he said benefited him because "it's hard not to have a personality meltdown ... after about several hours in that suit. I'm calling up every therapeutic moment I can think of to just get through the day."
• Terrence Howard as James "Rhodey" Rhodes: A friend of Stark's and the liaison between Stark Industries and the United States Air Force in the department of acquisitions, specifically weapons development. Favreau cast Howard because he felt he could play War Machine in a sequel. Howard prepared for the role by visiting Nellis Air Force Base on March 16, 2007, where he ate with the pilots and observed HH-60 Pave Hawk rescue helicopters and F-22 Raptors. While Rhodes is roguish in the comics after he meets Stark, his previous role as a disciplinarian creates a dynamic tension with Stark's character. He is unsure whether Stark's actions are acceptable. "Rhodey is completely disgusted with the way Tony has lived his life, but at a certain point he realizes that perhaps there is a different way," Howard said. "Whose life is the right way: Is it the strict military life, or the life of an independent?" Howard and his father are Iron Man fans, partly because Rhodes was one of the few black superheroes when Howard was a child. Howard admired Downey as an actor since the latter appeared in Weird Science (1985); the two competed physically on set. Howard signed a three-picture deal with Marvel Studios.
• Jeff Bridges as Obadiah Stane: Stark's business second-in-command, mentor, and friend, who turns on him to take over the company, eventually building a giant exosuit to fight Stark. Bridges read the comics as a boy and liked Favreau's modern, realistic approach. He …

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In April 1990, Universal Pictures bought the rights to develop Iron Man for the big screen, with Stuart Gordon to direct a low-budget film based on the property. By February 1996, 20th Century Fox had acquired the rights from Universal. In January 1997, Nicolas Cage expressed interest in portraying the character, while in September 1998, Tom Cruise expressed interest in producing as well as starring in an Iron Man film. Jeff Vintar and Iron Man co-creator Stan Lee co-wrote a story for Fox, which Vintar adapted into a screenplay. It included a new science-fiction origin for the character, and featured MODOK as the villain. Tom Rothman, President of Production at Fox, credited the screenplay with finally making him understand the character. In May 1999, Jeffrey Caine was hired to rewrite Vintar and Lee's script. That October, Quentin Tarantino was approached to write and direct the film. Fox sold the rights to New Line Cinema the following December, reasoning that although the Vintar/Lee script was strong, the studio had too many Marvel superheroes in development, and "we can't make them all."

By July 2000, the film was being written for New Line Cinema by Ted Elliott, Terry Rossio, and Tim McCanlies. McCanlies' script used the idea of a Nick Fury cameo to set up his own film. In June 2001, New Line Cinema entered talks with Joss Whedon, a fan of the character, to direct, and in December 2002, McCanlies had turned in a completed script. New Line took a "unique" approach to writing the film's script, hiring David Hayter, David S. Goyer, and Mark Protosevich to "sit in a room and simply talk on camera about Iron Man for a few days". After this, Hayter was hired in 2004 to write a script. He reworked scripts that had been written by Jeff Vintar and Alfred Gough and Miles Millar, which had included the villain the Mandarin and Pepper Potts as a love interest. Hayter removed the Mandarin and instead chose to pit Iron Man against his father Howard Stark, who becomes War Machine. Hayter said "you want to try to mirror your hero with your villain as much as possible" for his reasoning behind making Howard the villain. He also made Bethany Cabe the film's love interest over Potts. In December 2004, the studio attached director Nick Cassavetes to the project for a target 2006 release. However, this deal ultimately fell through, and Iron Man's film rights returned to Marvel.

In November 2005, Marvel Studios worked to start development from scratch, and announced Iron Man as their first independent feature, because the character was their only major one not already depicted in live action. Paramount Pictures was announced as Marvel's distribution partner for Iron Man. According to associate producer Jeremy Latcham, "we went after about 30 writers and they all passed," saying they were uninterested in the project due to both the relative obscurity of the character and the fact that it was solely a Mar…

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Composer Ramin Djawadi had been a fan of the character Iron Man as a child, saying that he always liked superheroes "that actually don't have any superpowers". After Favreau's previous collaborator John Debney was unavailable to score the film, Djawadi sought out the role himself. Favreau had a clear vision of heavy metal music and guitars for the project, saying that Tony Stark was more of a rock star than a traditional superhero. Djawadi subsequently composed most of the film's score on guitar, before arranging it for orchestra. Djawadi had help with arrangements and additional cues from Hans Zimmer and Remote Control Productions, and Rage Against the Machine guitarist Tom Morello, who also makes a cameo appearance in the film, contributed guitar performances to the score. The film also features a big band-style arrangement of the Iron Man theme song from the 1966 cartoon The Marvel Super Heroes from frequent Favreau collaborators John O'Brien and Rick Boston. A soundtrack featuring Djawadi's score was released by Lions Gate Records on April 29, 2008.