Sterling Hayden is absolutely amazing in this movie.
He's so unpredictable and feral in it, he's just like a force of nature. He's one of those actors who always just seemed to have "it" in terms of presence and intensity, but didn't seem to be that naturally talented but incredibly effective when he's in a well placed role. Before I had seen this movie I hadn't really noticed how funny he is in The Godfather which is by all rights a pretty austere movie, "You gotta go, you gotta go"
There are some actors who have incredible talent but won't ever have one role as good as his work in this film, The Godfather, and Dr. Strangelove.