After watching all of Kennedy's supporting nominations I must say he is not in the league of Claude Rains, and in fact most of his nominations are pretty standard supporting performance that have absolutely no real distinction to them. This is probably the most true about his performance here though as Frank Sinatra's character's brother. The brother who tries his best to defend his brother, and tries to talk to him but has very little success either way. His whole character does feel a bit dispensable, particularly when the film attempts to give him a sub plot involving affair with his secretary caused by the coldness of his wife.
His character is rather repetitive and by the end of the film one could care less about him. Kennedy's follows suit with his performance which is equally repetitive. His performance is basically a one note of awkwardness around his defense of his character's brother. There is not any special connection ever shown between him and Sinatra in fact the way they were together you'd think they weren't brothers at all. After all there should be some small history between the two even if a small one, but the way the two interact suggests absolutely nothing. It does not even suggest estrangement even, since Kennedy plays his part in such a superficial fashion throughout his performance.
It just a terribly uninteresting performance for a terribly uninteresting character. Even when he has his affair, which the film barely cares about, Kennedy still does not make anything out of Frank. He still stays on the same basic note, and the affair just feels like something written for him to do rather than passionate action by Frank. I suppose the fact that Frank is boring does have a great deal in the way the part is written, really Kennedy has very little to work with to be perfectly honest, but still there was probably something more one could have done with the part than Kennedy does. This is not to say that Kennedy truly awful in the part, but at the same time it is a very dull performance from an actor seemed better suited for lead roles, at least when it comes to his Oscar nominated performances.