Kazan’s great casting hunch was James Dean, a young actor who had played the tantalizing Arab boy in a stage adaptation of André Gide’s The Immoralist and succeeded in seducing critics in addition to the play’s protagonist. Screenwriter Paul Osborn suggested Dean, and after a brief interview, Kazan was sure this guy was Cal. Steinbeck agreed, “said he sure as hell was, and that was it”.

Robert Cornfield, in Elia Kazan, Kazan on Directing, Alfred A. Knopf, New York 2009

I felt that Dean’s body was very graphic; it was almost writhing in pain sometimes […]. He even walked like a crab, as if he were cringing all the time. I felt that, and that doesn’t come across in close-up. Dean was a cripple, anyway, inside – he was not like Brando. People compared them, but there was no similarity. He was a far, far sick er kid, and Brando’s not sick, he’s just troubled. But I also think there was a value in Dean’s face. His face is so desolate and lonely and strange. And there are moments in it when you say, “Oh, God, he’s handsome – what’s being lost here! What goodness is being lost here!”
Dean had the most natural talent after Marlon. But he lacked technique: he had no proper training. He could not play a part outside his range. He often hit a scene immediately and instinctively right. Sometimes he did not and then there would be problems. Nor was his intelligence of a high order. Directing him was gratifying because he always caught something of the spirit of the youth which considered itself disenfranchised by the preceding generation, their parts. But there was an element of self-pity here and I found this irksome. He had considerable innocence but not as an adjunct of strength or courage; but of hatred and a kind of despair. His imagination was limited; it was like a child’s. To direct him was somewhat like directing Lassie the dog; the director dealt in a series of rewards and threats and played a psychological game with him. He had to be coddled and hugged or threatened with abandonment. His own favorite actor was Brando, and the only word possible here is hero-worship. When Brando came to visit my set of East of Eden, Jimmy was awestruck and nearly shriveled with respect. But I cannot concur in an impression that has some currency, that he fell into Marlon’s mannerisms; he had his own and they were ample.

Elia Kazan, Kazan on Directing, Alfred A. Knopf, New York 2009