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Hammer's factory is really Elon Musk's SpaceX facility in Hawthorne, California. The people walking in the background are actual employees, even though filming took place at night.
Scarlett Johansson dyed her hair red before she even got the part of Natasha Romanoff, a.k.a. Black Widow, because she wanted the role so badly.
A lot of Whiplash's identifying features were suggested by Mickey Rourke. He wanted to perform half of his role in Russian, and consulted on the character's tattoos and gold teeth, as well as having a pet cockatoo. In fact, Rourke paid for the bird and the gold teeth out of his own pocket.
According to director Jon Favreau, the technology in this film was portrayed as more futuristic: "After the first film, a number of tech companies talked about how uncanny a lot of our depictions of technology had turned out to be, and how many different films and videogames ended up being inspired by the imagery we used. This forced us to go a bit further into the future, and try and change the nature of this technology. If we'd just duplicated what happened in the first one, we would be behind the curve. So now we're dealing with holograms, the interface within the suit, and the suit being upgraded too."
Numerous news clippings shown in the film show Tony Stark and Ivan Vanko when they were younger men. Most of these are actual photos of Robert Downey Jr. and Mickey Rourke, both of whom came to fame in the 1980s.
Jon Favreau had a lot of friction with the Marvel higher-ups due to their constant intervention, to the point that they were having him rewrite the script as the film was still shooting. In particular, elements such as the increased prominence of the S.H.I.E.L.D. subplot were the result of a need to establish the greater Marvel Cinematic Universe, in preparation for The Avengers (2012), at the expense of the coherence of the film's own plot. These disputes got so bad that Favreau turned down directing Iron Man 3 (2013).
DJ AM: (at around 53 mins) The D.J. at Tony's birthday party is DJ-AM, who died after principal photography had wrapped, making this his last film project. During the end credits, the film is dedicated to him.
Seth Green: (at around 10 mins) While Tony is leaving the Expo and meets "Larry King" and Larry Ellison. Seth Green made several spoofs of Iron Man (including "Little Iron Man") in Robot Chicken (2005). Green made another cameo in the Marvel Cinematic Universe as the voice of Howard the Duck during the post-credits scene in Guardians of the Galaxy (2014).
Larry Ellison: (at around 10 mins) The CEO of Oracle Corporation (a billionaire playboy, who has often been compared to Tony Stark) is glimpsed briefly at the Stark Expo. While leading Tony out of the Stark Expo, Happy Hogan (Jon Favreau) says, "This is Larry." Stark replies, "Hey, the Oracle of Oracle." Oracle's brand is prominently placed at several points in the film, including the climactic showdown at a fictional "Oracle Biodome."