She was a naive innocent girl that didn't know what love was and didn't know the difference between sex/love/lust. He was a mature, sophisticated man of high power and lots of money. At the end, he gave her a massive diamond ring, and she caved. Definitely a cautionary tale.
Tony Leung is THE best. That scene of her singing to him in the Japanese restaurant, and him crying, it was simply gut-wrenching.
Math Teacher really liked the spy part of it, but he couldn't stand the stupidity of the college students.
I've shown Math Teacher both Ang Lee's movies, Brokeback Mountain and Lust/Caution. Both started slow; both had minimum characters but lots personal development; both had sex scenes. I promised him this was the last artsy movie from Ang Lee that I'd show him.