Well... I found this occasionally smile-ish, but not particularly chucklesome (sorry, F). The times when the super serious, white shirted, dark suited and hatted FBI team is planning and executing a complicated surveillance on... yes, a cat, come the closest to being truly funny.
An acting tour de force on all fronts, but I think Ralph Richardson's performance stands out as one of the best I've seen. His character's dismay, revulsion, frustration, and crushing disappointment are tightly bound by his commitment to decorum and politeness, but flash out through his steely gaze anyway. Every sarcastic comment reveals a ferocious intelligence at work. He has created what he despises, and reaps the harvest. Is the end a triumph of sorts, or a terrible tragedy? The always superb William Wyler gives this his sure directorial hand, and the beautifully lit and composed layered camera shots are textbook perfect.