J. Robert Oppenheimer
Played by Cillian Murphy, J. Robert Oppenheimer is the novel's main character and protagonist. A painfully shy but brilliant man, viewers first meet Oppenheimer in 1926 when he was a 22-year-old graduate student at Cambridge in England. He is portrayed as an enigmatic figure who few seem to notice. But when he interacts with people, like his tutor, he often clashes with him. In fact, Oppenheimer (his friends call him "Oppie") poisons his tutor's apple, hoping to kill him. He ultimately doesn't follow through with the plan, but this incident showcases his desire to seek revenge for what he perceives as mistreatment. It also foreshadows his crusade later in life against nuclear weapons after being slighted by President Truman.
Viewers see Oppenheimer navigate his early life in Cambridge and later, in Germany, where he completes his master's degree before Hitler's rise. In Germany, Oppenheimer is tasked with giving a lecture to students. He didn't know German before, but to prepare for the lecture (which everyone expected to be given in German), he learns how to speak German, reflecting his fierce intelligence and commitment to get things right.
The film portrays Oppenheimer as a hard worker. Through thick and thin, he perseveres and against all odds, leads the Manhattan Project to success. He and his team ultimately create an atomic weapon, which was later deployed against the Japanese during World War II. To do so, he was forced to navigate tricky political situations and people who were less-than-amenable to making his life easy. Throughout his life, he was also forced to contend with difficulties in his personal life, particularly his love life. He engaged in a number of affairs throughout his life, including with Jean Tatlock, which is depicted in the film.
Later in life, the film shows Oppenheimer settling into a scientific life largely out of the public eye. In the late 1950s, however, he became involved in a scandal regarding his security clearance. Forces led by Admiral Lewis Strauss aimed to strip Oppenheimer of his security clearance because they believed he was affiliated to Communist organizations. After significant turmoil and a long hearing, his clearance was stripped, further disillusioning Oppenheimer from the government and governmental activities.
Kitty Oppenheimer
Emily Blunt plays Kitty Oppenheimer, Robert's devoted wife. Although she is not given much screen time in the film, she is portrayed as being a hyper loyal and intelligent, but oftentimes unstable, woman who was an avowed member of the Communist party. At the start of the film, she is in an unhappy marriage to another man. Because of this (and because he was attracted to her), Robert pursued Kitty and the two began a torrid love affair, which ultimately resulted in Kitty becoming pregnant with Robert's child. Kitty divorced her first husband, and quickly married Robert.
Their personal life was marked by periods of instability, brought about by Kitty's unhappiness and alcoholism, as well as Robert's affairs. Despite this, their relationship stood the test of time, and Kitty remained devoted to her husband despite his own troubles with the government.
Admiral Lewis Strauss
Played by Robert Downey Jr., Lewis Strauss was the director of the Atomic Energy Commission and is one of the film's main antagonists. He was a hot-tempered and manipulative man who initially got along with Robert Oppenheimer, offering him a cushy job at Princeton (and later, a spot alongside him on the Atomic Energy Commission). Politically, Strauss carried a significant amount of pull and wielded his power with an iron fist. After he and Oppenheimer had a series of disagreements about hydrogen bombs, he orchestrated a plot to revoke Oppenheimer's security clearance. Later, in the Dwight D. Eisenhower Administration, Strauss was nominated to be Secretary of Commerce. However, the Senate did not approve his nomination because of the aforementioned hearing and his feuds with Oppenheimer, angering Strauss and effectively ending his life in the public eye.
General Leslie Groves
Played by Matt Damon, General Leslie Groves was the hard-nosed, savvy military leader of the Manhattan Project. He personally recruited Oppenheimer to join the Manhattan Project, believing him to the best (and only) man for the job.
Groves, who later was called to testify during Oppenheimer's security clearance hearing, is portrayed as being a man interested in maintaining order by any means necessary, even if it means breaking well-established rules. Groves was chiefly responsible for ensuring the project -- and Oppenheimer -- had what it needed to successfully build an atomic weapon. He was successful, as he obtained enough radioactive material to allow for the creation of three weapons--two of which were used in the field.
