There's a precice moment in Before Sunset the masterpiece sequel to 1995's Before Sunrise when the film transforms into a classic screen romance. Midway through the movie, Ethan Hawke's Jesse and Julie Delpy's Celine are on a boat on the Seine, nine years after she missed their planned rendezvous in Austria. ''Ohhh God! Why weren't you there in Vienna?'' Jesse suddenly groans, finally cracking through the getting-to-know-you-again chitchat and shooting Delpy an unexpectedly angry sidelong look. ''I wish you woulda been our lives mighta been so much different.'' From there on, the conversation cuts into a plumper vein of confession, regret, and comedy, culminating in an out-of-nowhere last scene that's just about the most powerful, perfect movie ending since Chief Bromden busted out of the asylum in One Flew Over the Cuckoo's Nest. ''The whole goal,'' says Delpy, 35, ''is to have the feeling that you're peeling an onion emotionally.'' Steadily notching up the intimacy as Jesse and Celine's day rolls on, Hawke and Delpy are subtle yet virtuosic; the actors make it look too easy. ''It's clear that people think it's me being myself,'' says Hawke, 34, a woefully underrated actor. ''I try to take that as a compliment.'' Here's another: Sunset, electrically wise and affecting, is a triumph for its hardworking actors. And for movie love!