Robert Downey Jr. plays Tony Stark / Iron Man: Favreau had planned to cast a newcomer in the role, but ultimately chose Downey (a fan of the comic) because he felt the actor's past, including cocaine abuse, made him an appropriate choice for the part.

"The best and worst moments of Robert's life have been in the public eye," the director explained. "He had to find an inner balance to overcome obstacles that went far beyond his career. That's Tony Stark. Robert brings a depth that goes beyond a comic-book character who is having trouble in high school, or can't get the girl." Favreau also felt Downey could make Stark a "likable asshole", but also depict an authentic emotional journey once he won over the audience.

Downey had an office next to Favreau during pre-production, which allowed him greater involvement in the screenwriting process. He brought a deeper sense of humor to the film not present in previous drafts of the script. He 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 prepare, Downey spent five days a week weight training and practiced martial arts to get into shape, which benefitted 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 plays Lt. Colonel James "Rhodey" Rhodes: A pilot friend of Stark, and the liaison between Stark Industries and the military in the department of acquisitions. Favreau cast Howard because he felt he could play War Machine in a sequel. Howard and his father are Iron Man fans, partly because Rhodes was one of the few black superheroes when he was a child.

While Rhodes is roguish in the comics after he met Stark, his earlier disciplinarian character forms a dynamic with Stark, and 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 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. He was a Downey fan since he saw him in Weird Science, and they competed physically on set: "Robert and his competitive ass almost tore my shoulder trying to keep up with him. Because I'm forty or fifty pounds heavier than him, so I'm in there lifting and I pushed up about 225 and knocked it out ten times. Robert wanted to go about 235, and he did it. So I'm going to push it up to about 245. I took him out running and gave him some nice cramps. He couldn't walk after a couple of days."

Gwyneth Paltrow plays Virginia "Pepper" Potts: Stark's personal secretary and budding love interest. Paltrow asked Marvel to send her any comics that they would consider relevant to her understanding of the character, which she considered to be very smart, very levelheaded, grounded, and she liked "the fact that there's a sexuality that's not blatant." Jon Favreau wanted Potts' and Stark's relationship to be reminiscent of a 1940s comedy, something which Paltrow considered to be fun in a sexy yet innocent way.

Jeff Bridges plays Obadiah Stane: Second-in-command at Stark Industries, who wants control of the company. When Stark declares he'll no longer manufacture arms, Stane steals the blueprints for the Mark I armor to create his own, bigger version. Bridges read the comics as a boy and liked Favreau's modern, realistic approach. He shaved his hair and grew a gray beard for the role, which was something he had wanted to do for some time. Bridges googled the Book of Obadiah, and he was surprised to learn retribution is a major theme in that particular book of the Bible, something which Stane represents.

Shaun Toub plays Dr. Yinsen: Stark's fellow captive in Afghanistan. He is a scientist who has traveled the world, and aids Stark in creating the Mark I and keeping it a secret from their captors. He also acts as Stark's mentor, showing him humility during their time together, and his death being one of the principal reasons behind Stark's decision to change the direction of his company. In the comics, Yinsen was Chinese, but in the film, he comes from an Afghan village called Gulmira.

Faran Tahir plays Raza: A terrorist hired by Stane to capture Stark, who then orders Stark to build a Stark Industries missile system for his organization, the Ten Rings. Tahir is an Iron Man reader, and wanted to bring humanity to the henchman. "I tried to find ways to show that although he may be the bad guy, there might be a moment or just a hint of vulnerability at times, where he hasn't made the right calculations or there's a certain amount of doubt. Jon was very receptive to that kind of layering."

Paul Bettany voices Jarvis, Stark's personal AI, which assists him in the construction and programming of the Iron Man suit. The name of the character is a reference to the comic book character Edwin Jarvis, Stark's butler.

Leslie Bibb plays Christine Everhart: A Vanity Fair columnist with whom Stark shares a one-night stand before he leaves for Afghanistan. Later, she appears again, to tell Stark of the Ten Rings in Gulmira and at the end, suspecting Stark of being Iron Man.

Cameos include Iron Man co-creator Stan Lee (whom Stark mistakes for Hugh Hefner at a party), and director Jon Favreau as Stark's bodyguard/chauffeur Happy Hogan. Ghostface Killah cameoed in a scene where Stark briefly stays in Dubai while returning to Afghanistan, but it was cut for pacing reasons.

Hilary Swank reportedly had a cameo in the film, but this was revealed to be part of a disinformation campaign by Favreau designed to keep "website guessers at bay". Audioslave and Rage Against the Machine guitarist Tom Morello, who provides additional guitar music for the film, has a brief cameo as a guard. Following the credits, Samuel L. Jackson appears as Nick Fury, telling Stark about the "Avenger Initiative".

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