there's no one scene that really stands on its own as something you could pull out as representative of the whole thing. the heart of the movie is the love triangle between kathy, ruth, and tommy, so it'd have to be something involving them, and something that hints at the secrets surrounding them.

to me, it's the scene where kathy is listening to ruth and tommy having sex in the next room, then rolls over and suddenly ruth is standing there in the doorway. she goes to kathy's bed and tells her, "i know what you think, kathy," and proceeds to tell her she never had a chance with tommy. then follows that by telling her she knows why kathy was looking at the porno magazines, ending creepily with: "he didn't understand what you were doing. but i did."

the danger is that you could read that scene on its own and figure the secret is that kathy's a lesbian or ruth is secretly into her or something like that. the way ruth touches kathy's hair and whispers in her ear makes that the obvious conclusion. but if played with the right amount of creepiness (and i think knightley is up to the task), and if you already know they're clones (which we do at that point in the script), that scene tells you everything you need to know about the three main characters while making you wonder-- as i did-- what exactly kathy was doing with that magazine.