Saw a documentary, Don't Expect Too Much, about director Nicholas Ray teaching at a university somewhere. His students were excited about getting to make a feature film, but Ray thought that would be the perfect time to make a wildly experimental film. A little surprising considering how conventional his visual style was.
Ray said that he always loudly berated his assistant director the first day of production and that he never had to yell at anyone after that. But his students were afraid of him and alcoholism was taking its toll.
I always thought James Dean was a terrible actor. No human being behaves the way he acted. There was a scene in East of Eden that was the worst.
I've read quotes from Ray and Elia Kazan about Dean's limited acting ability.
Kazan wrote in his autobiography:
"Dean had no technique to speak of. When he tried to play an older man in the last reels of Giant, he looked like what he was: a beginner. . . . On [East of Eden], Jimmy would either get the scene right immediately, without any detailed direction . . . or he couldn't get it at all."(I took the quote from this article: http://brightlightsfilm.com/70/70jamesdean_sewell.php#.VHUkrskVeGs)