In East of Eden Dean's methods worked well in creating the odd creature that Cal Trask was meant to be, in Giant he was a great maverick man, and even though I did not believe him as a old man Dean definitely tried to become his character in that film. Dean's worst acted scene of his short career is found in the opening minutes of the film. The opening scene is where Jim is suppose to be drunk and finds himself in a police station where his parents have to come to pick him up. Dean doesn't seem drunk rather seems like man trying to use every acting tic he came up with all at once. His delivery of "you're tearing me apart" does not seem the cry of a confused teenager rather the attempt of an actor to make himself known.
To be fair to Dean though he let's up on his ACTING after that opening scene and begins to actually try to find his character rather than try to simply show of his acting chops. Stark is probably Dean's least complex role actually because he does not have the biblical level conflicts of East of Eden, or the transformation from a young man with great ambition to an lusty old man filled with hate in Giant. Instead we have Jim whose biggest problem at the beginning of the film is that his warmer father is dominated by his cold mother. Even as the film continues the film really is more about the extreme actions and problems of others and less about Jim Stark's problems which seem much more minor by comparison.
James Dean actually most of the time gives a fairly relaxed performance as Jim. Jim often is one of the calmer characters in the film, and for most of the film Dean relies on his most natural aspect of his onscreen persona. That is of course his charisma and Dean personal charm, which is all his own, is rather endearing in its own. In more scenes his performance really does work when he is just normally interacting with the other teenagers in his fairly off beat fashion. Dean makes it easy to see why some of bullies would take offense to him because of his style that is anything but conformity but in this style there is such a magnetic quality that it is very believable why Natalie Wood's and Sal Mineo's characters would be attracted to him.
The interesting thing is that there are some later scenes in the film where Jim is troubled emotionally like the opening scene of the film, but Dean actually delivers in these scenes. Dean tones it down considerably and his reactions to his parents is far more believable as his more subtle discontent seems more fitting for a someone who lives with them day after day than the over top business at the beginning. Dean stays with his character and in doing so gives the emotional breakdown of Jim at the end of the film the power it deserves, since he does not distract with random mannerisms. Even though I find his first scene of the film to be the worst scene of his short career, I actually do like this performance overall as he does give an effective performance once he gets past his indulgences.