Why would Musk show up in a superhero movie? Because this is really a movie about fictional genius billionaire Tony Stark, played by Robert Downey Jr., with a few tons of digital special effects thrown in. When the movie creators needed a real-life person to base the character on, Downey told director Jon Favreau, "We need to sit down with Elon Musk."
Musk is no stranger to Hollywood (he was executive producer for Thank You for Smoking) and Tesla Motors is no stranger to the Iron Man movie series. A Roadster made a brief appearance in the first movie. In the sequel, Musk plays himself in a Monaco restaurant and is the guy Stark wants to talk to about an electric jet. Wait, where is the line between fiction and reality, again?