In 1953, director Elia Kazan was looking for a substantive actor to play the emotionally complex role of 'Cal Trask', for screenwriter Paul Osborn's adaptation of John Steinbeck's 1952 novel East of Eden. The lengthy novel deals with the story of the Trask and Hamilton families over the course of three generations, focusing especially on the lives of the latter two generations in Salinas Valley, California, from the mid-19th century through the 1910s. In contrast to the book, the film script focused on the last portion of the story, predominantly with the character of Cal. Though he initially seems more aloof and emotionally troubled than his twin brother Aron, Cal is soon seen to be more worldly, business savvy, and even sagacious than their pious and constantly disapproving father (played by Raymond Massey) who seeks to invent a vegetable refrigeration process. Cal is bothered by the mystery of their supposedly dead mother, and discovers she is still alive and a brothel-keeping 'madam'; the part was played by actress Jo Van Fleet.  Before casting Cal, Elia Kazan said that he wanted "a Brando" for the role and Osborn suggested Dean, a relatively unknown young actor. Dean met with Steinbeck, who did not like the moody, complex young man personally, but thought him to be perfect for the part. Dean was cast in the role and on April 8, 1954, left New York City and headed for Los Angeles to begin shooting.  Much of Dean's performance in the film is unscripted, including his dance in the bean field and his fetal-like posturing while riding on top of a train boxcar (after searching out his mother in nearby Monterey).  The most famous improvisation of the film occurs when Cal's father rejects his gift of $5,000, money Cal earned by speculating in beans before the US became involved in World War I. Instead of running away from his father as the script called for, Dean instinctively turned to Massey and in a gesture of extreme emotion, lunged forward and grabbed him in a full embrace, crying. Kazan kept this and Massey's shocked reaction in the film. Dean's performance in the film foreshadowed his role as Jim Stark in Rebel Without A Cause. Both characters are angst-ridden protagonists and misunderstood outcasts, desperately craving approval from their fathers.  In recognition of his performance in East of Eden, Dean was nominated posthumously for the 1956 Academy Awards as Best Actor in a Leading Role of 1955, the first official posthumous acting nomination in Academy Awards history. (Jeanne Eagels was nominated for Best Actress in 1929, when the rules for selection of the winner were different.) East of Eden was the only film starring Dean that he would see released in his lifetime.

Answer this question "What role did he play?" by extracting the answer from the text above.
Much of Dean's performance in the film is unscripted, including his dance in the bean field and his fetal-like posturing while riding on top of a train boxcar