Cillian Murphy
Cillian Murphy plays J. Robert Oppenheimer, the film's main character and protagonist. Murphy previously worked with director Christopher Nolan, having worked together on five films prior to Oppenheimer. Nolan, who also acted as the film's writer, wrote the film with Murphy in mind. Nolan is famous for not sending his scripts to his actors. Instead, he travels to them and encourages them to read his script while they are together. After getting offered the role, Murphy enthusiastically agreed to star in the film having not yet read the script. However, after reading the script, Murphy was relieved, believing it to be well-written and interesting.
To prepare for the role, Murphy dove headfirst into Oppenheimer's history. He meticoulously researched the man and spoke to Kip Thorne, who was Oppenheimer's graduate student and had extensive experience working with him. Said Murphy: "[Oppenheimer's personal life] and the hearing where he lost his security clearance and all of that, and how he kind of became vilified in American society, even though he had been this ... the most famous scientist in the world [was news to me]." He likewise lost weight so that he could match Oppenheimer's gaunt appearence, reflecting his overall demeanor.
Murphy's performance, which earned him $10 million, earned him critical praise. He later won an Academy Award for Best Actor, the first such win of his career.
Emily Blunt
Emily Blunt plays Katherine "Kitty" Oppenheimer, J. Robert Oppenheimer's fiery and devoted wife who supports him through thick and thin. As with her costars, the casting process for the film was shrouded in secrecy. Blunt knew little about the film she auditioned for and was only able to read the script in Nolan's presence.
Much like her costars, Blunt knew little about Kitty's story before agreeing to star in the film. To prepare, she read the book American Prometheus, the nonfiction book Oppenheimer is based on, and learned more about the woman. Nolan emphasized that each actor should become intimately familiar with the characters they played. And Blunt took that direction to heart, working hard to learn more about Kitty. Her hard work paid off, as her portrayal earned her an Academy Award nomination for Best Supporting Actress, her first such nod in her career.
Robert Downey Jr.
When Robert Downey Jr. was offered the role of Rear Admiral Lewis Strauss, who led the Atomic Energy Commission (a governing body that regulated the use of atomic energy in the United States) and was the person chiefly responsible for organizing an effort to strip Oppenheimer's security clearance from him, he asked to read the film's script. Nolan agreed and invited Downey to travel to his house, where he read the script, which was printed on red paper with black ink. He hurriedly accepted the role, later believing it to be the best film he has ever been a part of.,
Downey, who has in recent years become known for playing superhero Tony Stark/Iron Man, would play against type as Strauss, a curmudgeonly figure who resented Oppenheimer for publicly opposing his plans. Like his castmates, Downey's performance was widely praised, earning him an Academy Award for Best Supporting Actor.
Matt Damon
Matt Damon plays General Leslie Groves, the military leader of the Manhattan Project who enlists Oppenheimer's help to develop the atomic bomb. During the press tour for the film, Damon recounted agreeing with his wife that he would take a break from acting unless he got an offer from Christopher Nolan, who he had worked with on Interstellar. An offer came and Damon quickly accepted, technically fulfilling his promise to his wife.
Damon agreed to take a substantial pay cut to star in the film, earning $4 million instead of his typical $10 million. Damon happily embodied the role of Groves, the tough-as-nails General who acts a foil to Murphy's even-keeled Oppenheimer.
