You know, I hate to sound like a yes gal, but I have spoken of In A Lonely Place so often here, especially my favorite quote, "He was born when she kissed him, lived while she loved him and died when she left him." It's a gorgeous line in and of itself, but when he says it to her, in the subtly frightening way he had and asks what she thinks of it, it's obsessiveness really starts to creep her out.
I just adore this film, it's one of my most favorites and I have to second the motion of awarding him an Oscar for this performance, without question.

Speaking of In A Lonely Place, I was listening to a talk show and they were discussing the "Craig's List killer". The question was, at what point does a fiancé stop defending the man she loves, except the guilt of someone she's slept next to for years and promised to spend her life with?