The movie succeeds mainly because of the brilliant acting ability of Ray Milland. His performance, which owes much of its flavour to his Oscar winning role in "The Lost Weekend," is quite probably his best ever. Dialogue would have destroyed this movie because its atmosphere thrives on the solitude and loneliness of spies and their world.
I have read many reviews that mention that "The Thief" leaves unclear the political convictions of the protagonist as well as the name of the country for which he actually works. Why the emphasis on this I do not understand since the movie intentionally leaves so many things unclear (i.e.: Ray Milland is the only character whose name we learn). I feel that this works decidedly to the movies' advantage. Isn't that the very nature of espionage?