"You don't know what's on the desk."
"Paper."
Possibly my favourite line-reading in the history of film -- one word, and furthermore Burton's off-screen at the time.
While it's not hard to understand why so many people have difficulty making sense of the mechanics of the plot, for me, this film succeeds because the character of Leamas is so damned compelling. He is every inch the embittered, used-up, past-his-shelf-life operative -- part noir antihero, part conflicted cog, part soothsayer.
It's just the perfect marriage of character and actor; his face, as is, does most of the work.
I don't think it hurts that Burton, apart from a handful of film roles (including this one), was in very much the same position as his character -- being put to dubious use in vehicles not worth his time and expertise; he was also feeling the effects of years of hard drinking, and cynical about filmmaking (spycraft).
So, although I love everything about this movie, it would continue to work for me if it all came down to the cinematography and Burton's performance.