<Li> Robert Downey Jr. as Tony Stark / Iron Man: The benefactor of the Avengers, who is a self - described genius, billionaire, playboy, and philanthropist with electromechanical suits of armor of his own invention . On how his character evolves after the events of Iron Man 3, Downey said, "I think he realizes that tweaking and making all the suits in the world--which is what he has been doing--still didn't work for that thing of his tour of duty that left him a little PTSD . So his focus is more on how can we make it so that there's no problem to begin with . That, you know, there's a bouncer at our planet's rope . That's the big idea ." </Li> <Li> Chris Hemsworth as Thor: An Avenger and the crown prince of Asgard, based on the Norse mythological deity of the same name . Regarding Thor's place in the film, Hemsworth stated that as Thor has remained on Earth since Thor: The Dark World, and has begun to feel at home here, he considers Ultron's threat a personal attack . Hemsworth stated that he had to work harder to bring new elements to the character to avoid repeating himself saying, "It gave us room to kind of make him a little more grounded and human and have him in some civilian clothes and mixing it up at a party ." Hemsworth noted that Thor's motivations in this film were completely different, as it was the first MCU film where he did not play against Tom Hiddleston's character of Loki . </Li> <Li> Mark Ruffalo as Bruce Banner / Hulk: An Avenger and a genius scientist who, because of exposure to gamma radiation, transforms into a monster when enraged or agitated . To prepare for the role, Ruffalo worked with motion capture performer Andy Serkis' The Imaginarium Studios . He stated that his character had grown since the previous film and was "a bit more complex". Ruffalo explained that a confrontation is brewing between Banner and the Hulk saying, "There's a very cool thing happening: Hulk is as afraid of Banner as Banner is afraid of Hulk...and they have got to come to peace somehow with each other ." While filming in London, Ruffalo said that Whedon still had not given him any of the Hulk's lines . Whedon later explained that he writes the Hulk's dialogue spontaneously, saying, "What makes the Hulk so hard to write is that you're pretending he's a werewolf when he's a superhero . You want it vice versa...So the question is, how has he progressed? How can we bring changes on what the Hulk does? And that's not just in the screenplay, that's moment to moment ." </Li> <Li> Chris Evans as Steve Rogers / Captain America: The leader of the Avengers and a World War II veteran, who was enhanced to the peak of human physicality by an experimental serum and frozen in suspended animation before waking up in the modern world . Evans stated that since the fall of S.H.I.E.L.D. in Captain America: The Winter Soldier, Rogers has been left to depend on his Avenger teammates without the structure of military life and is now "looking to understand where he belongs, not just as a soldier, as Captain America, but as Steve Rogers, as a person ." Evans said that he was able to maintain the strength he built up for Captain America: The Winter Soldier by working out up to an hour a day . Regarding Captain America's fighting style, Evans felt he did not want to take a step back from the skills shown in The Winter Soldier, making sure Rogers' fight style advanced, showing "a consistent display of strength" and having Rogers utilizing his environment . </Li>

Who plays the hulk in the avengers age of ultron