In 2007, Downey was cast as the title character in the film Iron Man, with director Jon Favreau explaining the choice by stating: "Downey wasn't the most obvious choice, but he understood what makes the character tick. He found a lot of his own life experience in 'Tony Stark'." Favreau insisted on having Downey as he repeatedly claimed that Downey would be to Iron Man what Johnny Depp is to the Pirates of the Caribbean series, a lead actor that could both elevate the quality of the film and increase the public's interest in it. For the role Downey had to gain more than 20 pounds of muscle in five months to look like he "had the power to forge iron."
Iron Man was globally released between April 30 and May 3, 2008, grossing over $585 million worldwide and receiving rave reviews which cited Downey's performance as a highlight of the film. As a result, both Downey and Favreau stated their interest in making an Iron Man trilogy. By October 2008, Downey had agreed to appear as Iron Man in two Iron Man sequels and The Avengers, featuring the superhero team that Stark joins, based on Marvel's comic book series The Avengers. He also made a small appearance as Iron Man's alter ego Tony Stark in the 2008 film The Incredible Hulk, as a part of Marvel Studios' attempt to depict the same Marvel Universe on film by providing continuity among the movies. Downey reprised the role again in Avengers: Age of Ultron, and will do so yet again in Captain America: Civil War in 2016.