Much of the hype about Ang Lee's Lust, Caution revolves around the explicit sex scenes and the accompanying rumours about whether Tony Leung and Wei Tang actually, y'know, did it. I'm deeply cynical about this sort of synthesis of prurience and marketing: unless you can see exactly what's going on (as in 9 Songs), the answer is invariably "no".
But this time, the gossip actually makes sense, because the plot (in Japanese-occupied Shanghai, amateur actress-cum-resistance-activist assumes new identity to seduce collaborationist official and lure him to his death) deals with similar questions of honesty and authenticity. The (anti-?) heroine, Wong Chia Chi, is rarely seen in a context in which she isn't performing: whether acting in an earnestly patriotic play, or in the role of Mak Tai Tai, the respectable businesswoman who plays mah jongg with the wife of the slippery Mr Yee.
As the principals rut acrobatically, the ambiguity sets in. Is she in love? Is she in lust? Is she pretending? Did she start pretending, then start to believe in her own performance? And if we don't really know what the character is doing, why do we presume that we know what the actress, Wei Tang, is up to?
This is especially important in the most disturbing scene - which seems to have been overlooked in all the hoo-ha about the brief appearance of Tony Leung's scrotum - when Yee subjects Wong to a vicious rape. At one point, she appears to be enjoying it, which is usually the point at which my instinctively laissez-faire attitude to the depiction of sex and violence on screen tends to slip (cf the assault on Susan George in Peckinpah's Straw Dogs). But by this stage we know that Wong is taking method acting to the limit, and it's the character she's playing that expresses delight in her own degradation; although we never really know what Wong herself feels. Which in turn reminds us that this is just a film, and Wei Tang is pretending to be Wong Chia Chi is pretending to be Mak Tai Tai.
Which takes us back to the original question of "did they or didn't they?" To which the answer must be that, even if they really did it, they were only pretending.